powerpc/powernv: Use MSI bitmap to manage IRQs

As Michael Ellerman mentioned, arch/powerpc/sysdev/msi_bitmap.c
already implemented bitmap to manage (alloc/free) MSI interrupts.
The patch intends to use that mechanism to manage MSI interrupts
for PowerNV platform.

Signed-off-by: Gavin Shan <shangw@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>