ARM: dts: am335x-sl50: Specify the device to be used for boot console output.

UART0 device is the device to be used for boot console output.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>