selftests/powerpc: Use PKEY_UNRESTRICTED macro

Replace literal 0 with macro PKEY_UNRESTRICTED where pkey_*() functions
are used in mm selftests for memory protection keys for ppc target.

Signed-off-by: Yury Khrustalev <yury.khrustalev@arm.com>
Suggested-by: Kevin Brodsky <kevin.brodsky@arm.com>
Reviewed-by: Kevin Brodsky <kevin.brodsky@arm.com>
Link: https://lore.kernel.org/r/20250113170619.484698-4-yury.khrustalev@arm.com


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>