ARM: dts: am437x-idk-evm: Mark MAC as having only one PHY

Currently am4372.dtsi declares the MAC controller to have two
slave ports, on this board we only use one, so set the slave
count to one. This eliminates a console error message when
the non-existent PHY is not detected.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Reviewed-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>