net: ethernet: ti: am65-cpsw: Add support for SERDES configuration

Use PHY framework APIs to initialize the SERDES PHY connected to CPSW MAC.

Define the functions am65_cpsw_disable_phy(), am65_cpsw_enable_phy(),
am65_cpsw_disable_serdes_phy() and am65_cpsw_enable_serdes_phy().

Add new member "serdes_phy" to struct "am65_cpsw_slave_data" to store the
SERDES PHY for each port, if it exists. Use it later while disabling the
SERDES PHY for each port.

Power on and initialize the SerDes PHY in am65_cpsw_nuss_init_slave_ports()
by invoking am65_cpsw_enable_serdes_phy().

Power off the SerDes PHY in am65_cpsw_nuss_remove() by invoking
am65_cpsw_disable_serdes_phy().

Signed-off-by: Siddharth Vadapalli <s-vadapalli@ti.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>