arm64: implement PKEYS support

Implement the PKEYS interface, using the Permission Overlay Extension.

Signed-off-by: Joey Gouly <joey.gouly@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20240822151113.1479789-19-joey.gouly@arm.com


Signed-off-by: Will Deacon <will@kernel.org>