The MPIC code can already perform an automatic OF address translation
step as part of mpic_alloc(), but several boards need to use that base
address when they perform mpic_assign_isu().

The easiest solution is to save the computed physical address into the
"struct mpic" for later use by the board code.
Signed-off-by: NKyle Moffett <Kyle.D.Moffett@boeing.com>
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>