net: stmmac: unify mdio functions

stmmac_mdio_{read|write} and stmmac_mdio_{read|write}_gmac4 are not
enought different for being split.
The only differences between thoses two functions are shift/mask for
addr/reg/clk_csr.

This patch introduce a per platform set of variable for setting thoses
shift/mask and unify mdio read and write functions.

Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/ethernet/stmicro/stmmac/common.h b/drivers/net/ethernet/stmicro/stmmac/common.h
index 5bd4c05..3ced2e1 100644
--- a/drivers/net/ethernet/stmicro/stmmac/common.h
+++ b/drivers/net/ethernet/stmicro/stmmac/common.h
@@ -507,6 +507,12 @@ struct mac_link {
 struct mii_regs {
 	unsigned int addr;	/* MII Address */
 	unsigned int data;	/* MII Data */
+	unsigned int addr_shift;	/* MII address shift */
+	unsigned int reg_shift;		/* MII reg shift */
+	unsigned int addr_mask;		/* MII address mask */
+	unsigned int reg_mask;		/* MII reg mask */
+	unsigned int clk_csr_shift;
+	unsigned int clk_csr_mask;
 };
 
 /* Helpers to manage the descriptors for chain and ring modes */