powerpc/mpic: Use the MPIC_LARGE_VECTORS flag for FSL MPIC.

We should use the MPIC_LARG_VECTORS flag while intializing the MPIC.
This prevents us from eating in to hardware vector number space (MSIs)
while setting up internal sources.

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