powerpc/corenet: Change criteria to set MPIC_ENABLE_COREINT

Don't use ppc_md function comparison.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/c8ef82ee5f2713f4c36eb5d2d49b0905c7472801.1630667612.git.christophe.leroy@csgroup.eu