arch/c6x/include/asm/pgtable.h: define dummy pgprot_writecombine for !MMU

When !MMU, asm-generic will not define default pgprot_writecombine, so c6x
needs to define it by itself.  The related error:

    CC [M]  fs/pstore/ram_core.o
  fs/pstore/ram_core.c: In function 'persistent_ram_vmap':
  fs/pstore/ram_core.c:399:10: error: implicit declaration of function 'pgprot_writecombine' [-Werror=implicit-function-declaration]
     prot = pgprot_writecombine(PAGE_KERNEL);
            ^
  fs/pstore/ram_core.c:399:8: error: incompatible types when assigning to type 'pgprot_t {aka struct <anonymous>}' from type 'int'
     prot = pgprot_writecombine(PAGE_KERNEL);
          ^
Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Cc: Mark Salter <msalter@redhat.com>
Cc: Aurelien Jacquiot <a-jacquiot@ti.com>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/c6x/include/asm/pgtable.h

@@ -67,6 +67,11 @@ extern unsigned long empty_zero_page;
  */
 #define pgtable_cache_init()   do { } while (0)
 
+/*
+ * c6x is !MMU, so define the simpliest implementation
+ */
+#define pgprot_writecombine pgprot_noncached
+
 #include <asm-generic/pgtable.h>
 
 #endif /* _ASM_C6X_PGTABLE_H */