ARM: dts: exynos: Fix memory on Artik5 evaluation boards

The last 8MB of physical memory on Artik520 family boards is reserved for
secure firmware. Adjust the total amount of the memory defined in
exynos3250-artik5.dtsi to match the memory available for the Linux kernel.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>