powerpc/85xx: Fix support for enabling doorbells for IPIs

Commit 765342526246c97600e5344c0949824d94bb51c3 made some small changes to
IPI, message_pass in smp_ops was initialized to NULL for other platforms
but not for 85xx which causes us to always use the mpic for IPI's even
if we support doorbells in HW.

Signed-off-by: Matthew McClintock <msm@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>