driver: net: ti: cpsw-mdio: use phys_addr_t for mdio_base addr

Use phys_addr_t for mdio_base address to avoid build
warnings on arm64 and dra7. Cast it to uintprt_t before
assigning to regs.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Keerthy <j-keerthy@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>