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>

arch/arm/boot/dts/am335x-boneblack-common.dtsi

@@ -131,6 +131,11 @@ &mcasp0	{
 };
 
 / {
+	memory@80000000 {
+		device_type = "memory";
+		reg = <0x80000000 0x20000000>; /* 512 MB */
+	};
+
 	clk_mcasp0_fixed: clk_mcasp0_fixed {
 		#clock-cells = <0>;
 		compatible = "fixed-clock";