Add a dependency on CONFIG_MMU to ofdrm. The driver uses GEM SHMEM
helpers, which require MMU support. A reported error message [1] is
shown below.

   arm-linux-gnueabi-ld: drivers/gpu/drm/drm_gem_shmem_helper.o: in function `drm_gem_shmem_fault':
   >> drivers/gpu/drm/drm_gem_shmem_helper.c:562: undefined reference to `vmf_insert_pfn'
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Fixes: c8a17756 ("drm/ofdrm: Add ofdrm for Open Firmware framebuffers")
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Javier Martinez Canillas <javierm@redhat.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Maxime Ripard <maxime@cerno.tech>
Cc: "Noralf Trønnes" <noralf@tronnes.org>
Cc: Danilo Krummrich <dakr@redhat.com>
Cc: Michal Suchanek <msuchanek@suse.de>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://lore.kernel.org/dri-devel/202210192029.ZFeJvqjv-lkp@intel.com/ # [1]
Link: https://patchwork.freedesktop.org/patch/msgid/20221101104049.15601-1-tzimmermann@suse.de