net: ethernet: ti: am65-cpsw: Update name of Serdes PHY

The bindings for the am65-cpsw driver use the name "serdes" to refer to
the Serdes PHY. Thus, update the name used for the Serdes PHY within the
am65_cpsw_init_serdes_phy() function from "serdes-phy" to "serdes".

Signed-off-by: Siddharth Vadapalli <s-vadapalli@ti.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>