fbdev: MIPS supports iomem addresses

fbdev: MIPS supports iomem addresses

Add MIPS to fb_* helpers list for iomem addresses. This silences Sparse
warnings about lacking __iomem address space casts:

drivers/video/fbdev/pvr2fb.c:800:9: sparse: sparse: incorrect type in argument 1 (different address spaces)
drivers/video/fbdev/pvr2fb.c:800:9: sparse:     expected void const *
drivers/video/fbdev/pvr2fb.c:800:9: sparse:     got char [noderef] __iomem *screen_base

Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/lkml/202210100209.tR2Iqbqk-lkp@intel.com/
Cc: Helge Deller <deller@gmx.de>
Cc: linux-fbdev@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Helge Deller <deller@gmx.de>