net: stmmac: modified pcs mode support for RGMII

snps databook noted that physical coding sublayer (PCS) interface
that can be used when the MAC is configured for the TBI, RTBI, or
SGMII PHY interface. we have RGMII and SGMII in a SoC and it also
has the PCS block. it needs stmmac_init_phy and stmmac_mdio_register
function for initializing phy when it used RGMII interface.

Signed-off-by: Dejin Zheng <zhengdejin5@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>