x86/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: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: x86@kernel.org
Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lore.kernel.org/r/20240822151113.1479789-3-joey.gouly@arm.com


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