powerpc/kuap: Make PPC_KUAP_DEBUG depend on PPC_KUAP only

PPC_KUAP_DEBUG is supported by all platforms doing PPC_KUAP,
it doesn't depend on Radix on book3s/64.

This will avoid adding one more dependency when implementing
KUAP on book3e/64.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/a5ff6228a36e51783b83d8c10d058db76e450f63.1634627931.git.christophe.leroy@csgroup.eu