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>