Call the MAPTI command earlier in the ITS driver

The GICv3 Software Overview suggests when allocating a new MSI/MSI-X
interrupt we need to call MAPD followed by MAPTI. Unfortunately the code
would place a MOVI command between these. This is invalid as it needs
values set by the MAPTI to be present.

Re-order so we allocate a temporary CPU for the interrupt, then use the
MAPTI command to assign the MSI to it.

MFC after: 2 weeks
Sponsored by: DARPA, AFRL