ARM: dts: correct memory size for exynos4412-odroidx

Last megabyte of RAM is used by secure firmware and should not be accessed
by Linux kernel, so correct available memory size in DTS file.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>