powerpc/mpic: Don't open-code dcr_resource_start

Don't open-code the OpenFirmware "dcr-reg" property lookup trying to map
DCR resources.  This makes the code a bit easier to read.

Signed-off-by: Kyle Moffett <Kyle.D.Moffett@boeing.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>