fsl-mc: msi: Added FSL-MC-specific member to the msi_desc's union

FSL-MC is a bus type different from PCI and platform, so it needs
its own member in the msi_desc's union.

Signed-off-by: J. German Rivera <German.Rivera@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>