[POWERPC] cell: Fix undefined reference to mmio_nvram_init

This fixes the following link error with CONFIG_PPC_CELL_NATIVE=y and
CONFIG_PPC_CELL_BLADE=n:

arch/powerpc/platforms/built-in.o: In function `.cell_setup_arch':
setup.c:(.init.text+0xe80): undefined reference to `.mmio_nvram_init'

Signed-off-by: Kou Ishizaki <Kou.Ishizaki@toshiba.co.jp>
Signed-off-by: Paul Mackerras <paulus@samba.org>