parisc: Implement __cpu_die() and __cpu_disable() for CPU hotplugging

Add relevant code to __cpu_die() and __cpu_disable() to finally enable
the CPU hotplugging features. Reset the irq count values in smp_callin()
to zero before bringing up the CPU.

It seems that the firmware may need up to 8 seconds to fully stop a CPU
in which no other PDC calls are allowed to be made. Use a timeout
__cpu_die() to accommodate for this.

Use "chcpu -d 1" to bring CPU1 down, and "chcpu -e 1" to bring it up.

Signed-off-by: Helge Deller <deller@gmx.de>
diff --git a/arch/parisc/Kconfig b/arch/parisc/Kconfig
index 590843b..52e550b 100644
--- a/arch/parisc/Kconfig
+++ b/arch/parisc/Kconfig
@@ -56,6 +56,7 @@
 	select HAVE_ARCH_TRACEHOOK
 	select HAVE_REGS_AND_STACK_ACCESS_API
 	select GENERIC_SCHED_CLOCK
+	select GENERIC_IRQ_MIGRATION if SMP
 	select HAVE_UNSTABLE_SCHED_CLOCK if SMP
 	select LEGACY_TIMER_TICK
 	select CPU_NO_EFFICIENT_FFS