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

commit b2f4fc34d31f33950a34a88b96849da31278ba44 from
git://git.ti.com/ti-linux-kernel/ti-linux-kernel.git

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

Define the functions am65_cpsw_init_phy(), am65_cpsw_enable_phy() and
am65_cpsw_disable_phy() and invoke in am65_cpsw_nuss_init_slave_ports(),
am65_cpsw_mac_link_up() and am65_cpsw_mac_link_down() respectively.

Signed-off-by: Siddharth Vadapalli <s-vadapalli@ti.com>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Signed-off-by: Xulin Sun <xulin.sun@windriver.com>