x86/Kconfig: Update config and kernel doc for MPK feature on AMD

AMD's next generation of EPYC processors support the MPK (Memory
Protection Keys) feature. Update the dependency and documentation.

Signed-off-by: Babu Moger <babu.moger@amd.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lkml.kernel.org/r/159068199556.26992.17733929401377275140.stgit@naples-babu.amd.com