Add support for MPIC timers as requestable interrupt sources.

Based on http://patchwork.ozlabs.org/patch/20941/ by Dave Liu.
Signed-off-by: NDave Liu <daveliu@freescale.com>
Signed-off-by: NScott Wood <scottwood@freescale.com>
Signed-off-by: NKumar Gala <galak@kernel.crashing.org>