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>

include/linux/fb.h

@@ -555,7 +555,7 @@ static inline struct apertures_struct *alloc_apertures(unsigned int max_num) {
 
 #elif defined(__i386__) || defined(__alpha__) || defined(__x86_64__) ||	\
 	defined(__hppa__) || defined(__sh__) || defined(__powerpc__) ||	\
-	defined(__arm__) || defined(__aarch64__)
+	defined(__arm__) || defined(__aarch64__) || defined(__mips__)
 
 #define fb_readb __raw_readb
 #define fb_readw __raw_readw