nds32: Fix drivers/gpu/drm/udl/udl_fb.c building error by defining PAGE_SHARED

It broke the 'allmodconfig' build.
drivers/gpu/drm/udl/udl_fb.c: In function 'udl_fb_mmap':
drivers/gpu/drm/udl/udl_fb.c:183:52: error: 'PAGE_SHARED' undeclared (first use in this function)
   if (remap_pfn_range(vma, start, page, PAGE_SIZE, PAGE_SHARED))
                                                    ^~~~~~~~~~~
drivers/gpu/drm/udl/udl_fb.c:183:52: note: each undeclared identifier is reported only once for each function it appears in
make[4]: *** [drivers/gpu/drm/udl/udl_fb.o] Error 1

Signed-off-by: Greentime Hu <greentime@andestech.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>