ARM: keystone: enable big endian support

Keystone code is big endian compatible,
so mark it as one that supports big endian.

Note this patch just allows to select Big endian build
for ARCH_KEYSTONE, but it does not enable BE by default.

Signed-off-by: Taras Kondratiuk <taras.kondratiuk@linaro.org>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>