xtensa: implement pgprot_noncached

The default pgprot_noncached doesn't do anything. This leads to issues
when drivers rely on it to disable caching in userspace mappings.
Implement pgprot_noncached properly so that caching of userspace mappings
could be controlled.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/include/asm/pgtable.h

@@ -277,6 +277,8 @@ static inline pte_t pte_mkwrite(pte_t pte)
 static inline pte_t pte_mkspecial(pte_t pte)
 	{ return pte; }
 
+#define pgprot_noncached(prot) (__pgprot(pgprot_val(prot) & ~_PAGE_CA_MASK))
+
 /*
  * Conversion functions: convert a page and protection to a page entry,
  * and a page entry and page directory to the page they refer to.