ARM: dts: imx53-m53evk: Fix memory region description

On m53evk there are two DRAM chip selects:

CS0 at 0x70000000
CS1 at 0xb0000000

Each bank has a 512MB DRAM, giving a total of 1GB of system DRAM.

Fix the memory layout to describe the hardware appropriately.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Marek Vasut <marex@denx.de>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>