powerpc/fsl_msi: enable msi allocation in all banks

Put all fsl_msi banks in a linked list. The list of banks then can be
traversed when allocating new msi interrupts.  Also fix failing path
of fsl_setup_msi_irqs().

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>