ARM: EXYNOS: Enable ARCH_SUPPORTS_BIG_ENDIAN explicitly

Now the initial fixes have the big-endian code working on EXYNOS, make sure
we explicitly mark our arch as being big endian capable.

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>