arm64: Track system support for mixed endian EL0

commit 04597a65c5efc207257a736d339c6f2f5b00250f upstream.

This patch keeps track of the mixed endian EL0 support across
the system and provides helper functions to export it. The status
is a boolean indicating whether all the CPUs on the system supports
mixed endian at EL0.

Signed-off-by: Suzuki K. Poulose <suzuki.poulose@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: David Brown <david.brown@linaro.org>