[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>
diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 901f12d..5f80f0b 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -491,6 +491,7 @@
 	select PPC_970_NAP
 	select PPC_NATIVE
 	select PPC_RTAS
+	select MMIO_NVRAM
 	default n
 	help
           This option enables support for the Maple 970FX Evaluation Board.