driver/net/fm/memac_phy: Initialize mdio_clock for SoCs wih FMANv3

MDIO clock needs to be initialized in u-boot code for SoCs
having FMAN-v3(v3H or v3L) controller due to below reasons

-On SoCs that have FMAN-v3H  like B4860, default value of
MDIO_CLK_DIV bits in mdio_stat(mdio_cfg) register generates
mdio clock too high (much higher than 2.5MHz), violating the
IEEE specs.
-On SOCs that have FMAN-v3L like T1040, default value of
MDIO_CLK_DIV bits is zero, so MDIO clock is disabled.

So, for proper functioninig of MDIO, MDIO_CLK_DIV bits needs to
be properly initialized.
Also this type of initialization is generally done in
PBI(pre-bootloader) phase using rcw.But for chips like T1040
which support deep-sleep, such type of initialization cannot be
done in PBI phase due to the limitation that during deep-sleep
resume, FMAN (MDIO) registers are not accessible in PBI phase.
So, mdio clock initailization must be done as part of u-boot.

This initialization code is implemented in memac_phy.c which
gets compiled only for SoCs having FMANv3, so no extra compilation
flag is required.

Signed-off-by: Priyanka Jain <Priyanka.Jain@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>