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>

arch/nds32/include/asm/pgtable.h

@@ -152,6 +152,7 @@ extern void __pgd_error(const char *file, int line, unsigned long val);
 #define PAGE_CACHE_L1	__pgprot(_HAVE_PAGE_L | _PAGE_V | _PAGE_M_KRW | _PAGE_D | _PAGE_E | _PAGE_G | _PAGE_CACHE)
 #define PAGE_MEMORY	__pgprot(_HAVE_PAGE_L | _PAGE_V | _PAGE_M_KRW | _PAGE_D | _PAGE_E | _PAGE_G | _PAGE_CACHE_SHRD)
 #define PAGE_KERNEL	__pgprot(_PAGE_V | _PAGE_M_KRW | _PAGE_D | _PAGE_E | _PAGE_G | _PAGE_CACHE_SHRD)
+#define PAGE_SHARED	__pgprot(_PAGE_V | _PAGE_M_URW_KRW | _PAGE_D | _PAGE_CACHE_SHRD)
 #define PAGE_DEVICE    __pgprot(_PAGE_V | _PAGE_M_KRW | _PAGE_D | _PAGE_G | _PAGE_C_DEV)
 #endif /* __ASSEMBLY__ */