powerpc/audit: Convert powerpc to AUDIT_ARCH_COMPAT_GENERIC

Commit e65e1fc2 ("[PATCH] syscall class hookup for all normal
targets") added generic support for AUDIT but that didn't include
support for bi-arch like powerpc.

Commit 4b588411

 ("audit: Add generic compat syscall support")
added generic support for bi-arch.

Convert powerpc to that bi-arch generic audit support.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/a4b3951d1191d4183d92a07a6097566bde60d00a.1629812058.git.christophe.leroy@csgroup.eu