powerpc/mpic: finish supporting timer group B on Freescale chips

Previously, these interrupts would be mapped, but the offset calculation
was broken, and only the first group was initialized.

Signed-off-by: Varun Sethi <Varun.Sethi@freescale.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>