powerpc/fsl_msi: show more meaningful names in /proc/interrupts

Rename the irq controller associated with a MSI
interrupt to fsl-msi-<V>, where <V> is the virq
of the cascade irq backing up this MSI interrupt.
This way, one can set the affinity of a MSI
through the cascade irq associated with said MSI
interrupt.
Given this example /proc/interrupts snippet:

           CPU0       CPU1       CPU2       CPU3
 16:          0          0          0          0   OpenPIC    16 Edge      mpic-error-int
 17:          0          4          0          0  fsl-msi-224   0 Edge      eth0-rx-0
 18:          0          5          0          0  fsl-msi-225   1 Edge      eth0-tx-0
 19:          0          2          0          0  fsl-msi-226   2 Edge      eth0
 [...]
224:          0         11          0          0   OpenPIC   224 Edge      fsl-msi-cascade
225:          0          0          0          0   OpenPIC   225 Edge      fsl-msi-cascade
226:          0          0          0          0   OpenPIC   226 Edge      fsl-msi-cascade
 [...]

To change the affinity of MSI interrupt 17
(having the irq controller named "fsl-msi-224")
instead of writing /proc/irq/17/smp_affinity, use
the associated MSI cascade irq, in this case,
interrupt 224, e.g.:

   echo 6 > /proc/irq/224/smp_affinity

Note that a MSI cascade irq covers several MSI
interrupts, so changing the affinity on the
cascade will impact all of the associated MSI
interrupts.

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>