mm/pkeys: Add an empty arch_pkeys_enabled()

Add an empty arch_pkeys_enabled() in linux/pkeys.h for the
CONFIG_ARCH_HAS_PKEYS=n case.

Split out of a patch by Ram Pai <linuxram@us.ibm.com>.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Dave Hansen <dave.hansen@intel.com>