ARM: dts: am335x-boneblack-common: fix memory size

BeagleBone Black series is equipped with 512MB RAM
whereas only 256MB is included from am335x-bone-common.dtsi

This leads to an issue with unusual setups when devicetree
is loaded by GRUB2 directly.

Signed-off-by: Matwey V. Kornilov <matwey@sai.msu.ru>
Signed-off-by: Tony Lindgren <tony@atomide.com>