net: ti: am65-cpsw-nuss: Get port mode register from standard "phys" property

Approved DT binding has the port mode register in the
"phys" property. Get it from there instead of the custom
"cpsw-phy-sel" property.

This will allow us to keep DT in sync with Linux.

Signed-off-by: Roger Quadros <rogerq@kernel.org>
Acked-by: Nishanth Menon <nm@ti.com>