blackfin: define dummy pgprot_writecombine for !MMU

blackfin allmodconfig build fails with the error:

  ../sound/core/pcm_native.c: In function 'snd_pcm_lib_default_mmap':
  ../sound/core/pcm_native.c:3386:24: error: implicit declaration of function 'pgprot_writecombine' [-Werror=implicit-function-declaration]
     area->vm_page_prot = pgprot_writecombine(area->vm_page_prot);
                          ^
  ../sound/core/pcm_native.c:3386:22: error: incompatible types when assigning to type 'pgprot_t {aka struct <anonymous>}' from type 'int'
     area->vm_page_prot = pgprot_writecombine(area->vm_page_prot);
                        ^

When !MMU, asm-generic will not define default pgprot_writecombine, so
blackfin needs to define it by itself.

The patch idea is from commit 65b9ab88 ("arch/c6x/include/asm/pgtable.h:
define dummy pgprot_writecombine for !MMU")
Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Cc: Steven Miao <realmz6@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/blackfin/include/asm/pgtable.h

@@ -97,6 +97,8 @@ extern unsigned long get_fb_unmapped_area(struct file *filp, unsigned long,
 					  unsigned long);
 #define HAVE_ARCH_FB_UNMAPPED_AREA
 
+#define pgprot_writecombine pgprot_noncached
+
 #include <asm-generic/pgtable.h>
 
 #endif				/* _BLACKFIN_PGTABLE_H */