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>