powerpc/features: Add capability to update mmu features later

On powerpc32, features fixup is performed very early and that's too
early to read the cmdline and take into account 'nosmap' parameter.

On the other hand, no userspace access is performed that early and
KUAP feature fixup can be performed later.

Add a function to update mmu features. The function is passed a
mask with the features that can be updated.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/31b27ee2c9d338f4f82cd8cd69d6bff979495290.1689091022.git.christophe.leroy@csgroup.eu