powerpc/64s: Replace RFI by RFI_TO_KERNEL and remove RFI

In head_64.S, we have two places using RFI to return to
kernel. Use RFI_TO_KERNEL instead.

They are the two only places using RFI on book3s/64, so
the RFI macro can go away.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Acked-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/7719261b0a0d2787772339484c33eb809723bca7.1604854583.git.christophe.leroy@csgroup.eu