[POWERPC] Generic DCR infrastructure

This patch adds new dcr_map/dcr_read/dcr_write accessors for DCRs that
can be used by drivers to transparently address either native DCRs or
memory mapped DCRs. The implementation for memory mapped DCRs is done
after the binding being currently worked on for SLOF and the Axon
chipset. This patch enables it for the cell native platform

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>