ARM: dts: am43x-epos-evm: Add Missing cpsw-phy-sel for am43x-epos-evm

AM437x EPOS evm use external clock for RMII interface.
Enable the same in DT.

Signed-off-by: George Cherian <george.cherian@ti.com>
Reported-by: Nishanth Menon <nm@ti.com>
Tested-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>