[POWERPC] Fix building of pmac32 when CONFIG_NVRAM=m

Kamalesh Babulal (kamalesh@linux.vnet.ibm.com) reports that CONFIG_NVRAM=m
is valid in terms of Kconfig but fails to build with:
  Building modules, stage 2.
  MODPOST 1401 modules
ERROR: "pmac_newworld" [arch/powerpc/platforms/powermac/nvram.ko] undefined!
ERROR: "__alloc_bootmem" [arch/powerpc/platforms/powermac/nvram.ko] undefined!
make[1]: *** [__modpost] Error

The arch/powerpc/platforms/powermac/nvram.c code really needs to be
builtin, but as its compilation is dependent on a generic Kconfig
symbol we force nvram.c to be builtin if CONFIG_NVRAM is 'y' or 'm'.

Signed-off-by: Tony Breeds <tony@bakeyournoodle.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>