powerpc/fsl_msi: reorganize structs to improve clarity and flexibility

Store cascade_data in an array inside the driver
data for later use.
Get rid of the msi_virq array since now we can
encapsulate the virqs in the cascade_data
directly and access them through the array
mentioned earlier.

Signed-off-by: Laurentiu Tudor <Laurentiu.Tudor@freescale.com>
Cc: Scott Wood <scottwood@freescale.com>
Cc: Mihai Caraman <mihai.caraman@freescale.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>