ARM: dts: reduce total RAM by 22 MiB for exynos5422-odroidxu3

Last 22 MiB is RAM is reserved by secure monitor code and cannot be
accessed from Linux kernel, so adjust total RAM size to 0x7EA00000
(2 GiB - 22 MiB). This fixes random 'imprecise kernel abort' kernel
failures.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Signed-off-by: Kukjin Kim <kgene@kernel.org>