powerpc: Consolidate mpic_alloc() OF address translation

Instead of using the open-coded "reg" property lookup and address
translation in mpic_alloc(), directly call of_address_to_resource().
This includes various workarounds for special cases which the naive
of_address_translate() does not.

Afterwards it is possible to remove the copiously copy-pasted calls to
of_address_translate() from the 85xx/86xx/powermac platforms.

Signed-off-by: Kyle Moffett <Kyle.D.Moffett@boeing.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>