video: fbdev: controlfb: fix build for COMPILE_TEST=y && PPC_PMAC=y && PPC32=n

video: fbdev: controlfb: fix build for COMPILE_TEST=y && PPC_PMAC=y && PPC32=n

powerpc allyesconfig fails like this:

drivers/video/fbdev/controlfb.c: In function 'controlfb_mmap':
drivers/video/fbdev/controlfb.c:756:23: error: implicit declaration of function 'pgprot_cached_wthru'; did you mean 'pgprot_cached'? [-Werror=implicit-function-declaration]
  756 |   vma->vm_page_prot = pgprot_cached_wthru(vma->vm_page_prot);
      |                       ^~~~~~~~~~~~~~~~~~~
      |                       pgprot_cached
drivers/video/fbdev/controlfb.c:756:23: error: incompatible types when assigning to type 'pgprot_t' {aka 'struct <anonymous>'} from type 'int'

Fix it by adding missing PPC32 dependency.

Fixes: a07a63b0e24d ("video: fbdev: controlfb: add COMPILE_TEST support")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reported-by: kbuild test robot <lkp@intel.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/fe520316-3863-e6c4-9581-5d709f49e906@samsung.com