ARM: dts: k2g-evm: Add unit address to memory node

With the new Keystone 2 Industrial Communication EVM adding the
unit address to the memory node it made sense to add it for this board
also.

Signed-off-by: Franklin S Cooper Jr <fcooper@ti.com>
Signed-off-by: Santosh Shilimkar <ssantosh@kernel.org>