ARM: dts: Fix TI CPSW Phy mode selection on IGEP COM AQUILA.

As this board use external clock for RMII interface we should specify 'rmii'
phy mode and 'rmii-clock-ext' to make ethernet working.

Signed-off-by: Enric Balletbo i Serra <eballetbo@iseebcn.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>