powerpc/fsl_msi: fix the conflict of virt_msir's chip_data

In fsl_of_msi_probe(), the virt_msir's chip_data have been stored
the pointer to struct mpic. We add a struct fsl_msi_cascade_data
to store the pointer to struct fsl_msi and msir_index in hanler_data.
Otherwise, the pointer to struct mpic will be over-written, and will
cause problem when calling eoi() of the irq.

Signed-off-by: Zhao Chenhui <b26998@freescale.com>
Signed-off-by: Li Yang <leoli@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>