ARM: dts: chromecast: override bad bootloader memory info

On the Chromecast, the bootloader provides us with an ATAG_MEM of
start=0x01000000 and size=0x3eff8000. This is clearly incorrect, as the
range given encompasses nearly a GiB but the Chromecast only has 512MiB
of RAM! Additionally, this causes the kernel to be decompressed at
0x00008000, below the claimed beginning of RAM, and so the boot fails.

Since the existing ATAG parsing code runs before the kernel is even
decompressed and irrevocably patches the device tree, don't even try
to bypass it. Instead, use the "linux,usable-memory" property instead
of the "reg" property to define the real range. The ATAG code only
overwrites reg, but linux,usable-memory is checked first in the OF
driver, so the fact that reg gets changed makes no difference.

Signed-off-by: Thomas Hebb <tommyhebb@gmail.com>
Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>