[POWERPC] Cleanup calling mmio_nvram_init

This makes mmio_nvram_init() callable unconditionally by providing
a dummy definition when CONFIG_MMIO_NVRAM is not defined.

Signed-off-by: Kou Ishizaki <Kou.Ishizaki@toshiba.co.jp>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>