[POWERPC] Maple: use mmio nvram

Some systems supported by the maple platform (e.g. JS2x blades running
SLOF) are able to use the mmio_nvram backend for reading and writing
nvram.  This is an improvement over the current situation -- no nvram
access from userspace at all.

Select MMIO_NVRAM for the maple platform.

Initialize the mmio_nvram backend from maple setup code.

Signed-off-by: Nathan Lynch <ntl@pobox.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>