Revert "powerpc/fsl_msi: spread msi ints across different MSIRs"

This reverts commit c822e73731fce3b49a4887140878d084d8a44c08.

This commit conflicted with a bitmap allocator change that partially
accomplishes the same thing, but which does so more correctly.  Revert
this one until it can be respun on top of the correct change.

Signed-off-by: Scott Wood <scottwood@freescale.com>