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 <[e-mail]>
Cc: Mark Salter <[e-mail]>
Cc: Aurelien Jacquiot <[e-mail]>
Cc: "Kirill A. Shutemov" <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

arch/c6x/include/asm/pgtable.h

···
  */
 #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 */