Broadcom BCM7120-style Level 2 interrupt controller

This interrupt controller hardware is a second level interrupt controller that
is hooked to a parent interrupt controller: e.g: ARM GIC for ARM-based
platforms. It can be found on BCM7xxx products starting with BCM7120.

Such an interrupt controller has the following hardware design:

- outputs multiple interrupts signals towards its interrupt controller parent

- controls how some of the interrupts will be flowing, whether they will
  directly output an interrupt signal towards the interrupt controller parent,
  or if they will output an interrupt signal at this 2nd level interrupt
  controller, in particular for UARTs

- has one 32-bit enable word and one 32-bit status word

- no atomic set/clear operations

- not all bits within the interrupt controller actually map to an interrupt

The typical hardware layout for this controller is represented below:

2nd level interrupt line		Outputs for the parent controller (e.g: ARM GIC)

0 -----[ MUX ] ------------|==========> GIC interrupt 75
          \-----------\
                       |
1 -----[ MUX ] --------)---|==========> GIC interrupt 76
          \------------|
                       |
2 -----[ MUX ] --------)---|==========> GIC interrupt 77
          \------------|
                       |
3 ---------------------|
4 ---------------------|
5 ---------------------|
7 ---------------------|---|===========> GIC interrupt 66
9 ---------------------|
10 --------------------|
11 --------------------/

6 ------------------------\
                           |===========> GIC interrupt 64
8 ------------------------/

12 ........................ X
13 ........................ X 		(not connected)
..
31 ........................ X

Required properties:

- compatible: should be "brcm,bcm7120-l2-intc"
- reg: specifies the base physical address and size of the registers
- interrupt-controller: identifies the node as an interrupt controller
- #interrupt-cells: specifies the number of cells needed to encode an interrupt
  source, should be 1.
- interrupts: specifies the interrupt line(s) in the interrupt-parent controller
  node, valid values depend on the type of parent interrupt controller
- brcm,int-map-mask: 32-bits bit mask describing how many and which interrupts
  are wired to this 2nd level interrupt controller, and how they match their
  respective interrupt parents. Should match exactly the number of interrupts
  specified in the 'interrupts' property.

Optional properties:

- brcm,irq-can-wake: if present, this means the L2 controller can be used as a
  wakeup source for system suspend/resume.

- brcm,int-fwd-mask: if present, a bit mask to configure the interrupts which
  have a mux gate, typically UARTs. Setting these bits will make their
  respective interrupt outputs bypass this 2nd level interrupt controller
  completely; it is completely transparent for the interrupt controller
  parent. This should have one 32-bit word per enable/status pair.

Example:

irq0_intc: interrupt-controller@f0406800 {
	compatible = "brcm,bcm7120-l2-intc";
	interrupt-parent = <&intc>;
	#interrupt-cells = <1>;
	reg = <0xf0406800 0x8>;
	interrupt-controller;
	interrupts = <0x0 0x42 0x0>, <0x0 0x40 0x0>;
	brcm,int-map-mask = <0xeb8>, <0x140>;
	brcm,int-fwd-mask = <0x7>;
};