powerpc/32s: Allow disabling KUAP at boot time

PPC64 uses MMU features to enable/disable KUAP at boot time.
But feature fixups are applied way too early on PPC32.

Now that all KUAP related actions are in C following the
conversion of KUAP initial setup and context switch in C,
static branches can be used to enable/disable KUAP.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
[mpe: Export disable_kuap_key to fix build errors]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/cd79e8008455fba5395d099f9bb1305c039b931c.1622708530.git.christophe.leroy@csgroup.eu