ARM: am335x: Add missing memory node in sl50 board

The skeleton.dtsi file was removed in ARM64 for different reasons as
explained in commit ("3ebee5a2

 arm64: dts: kill skeleton.dtsi").

These also applies to ARM and it will also allow to get rid of the
following DTC warnings in the future:

"Node /memory has a reg or ranges property, but no unit name"

But the sl50 board doesn't have a, so removing the  skeleton.dtsi
inclusion from am33xx.dtsi will cause a change in the compiled DTB.

The board has 512 MiB of RAM and its starting address is 0x80000000,
so add a proper memory device node in the DTS.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>