powerpc/mm: add ARCH_PKEY_BITS to Kconfig

powerpc/mm: add ARCH_PKEY_BITS to Kconfig

The new config option specifies how many bits are in each PKEY.

Signed-off-by: Joey Gouly <joey.gouly@arm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Aneesh Kumar K.V <aneesh.kumar@kernel.org>
Cc: Naveen N. Rao <naveen.n.rao@linux.ibm.com>
Cc: linuxppc-dev@lists.ozlabs.org
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20240822151113.1479789-2-joey.gouly@arm.com
Signed-off-by: Will Deacon <will@kernel.org>