powerpc: Add pgprot_writecombine

A number of drivers are using pgprot_writecombine() to enable write
combining on userspace mappings. Implement it on powerpc.
Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/include/asm/pgtable.h

@@ -170,6 +170,7 @@ extern int ptep_set_access_flags(struct vm_area_struct *vma, unsigned long addre
 #define pgprot_cached_wthru(prot) (__pgprot((pgprot_val(prot) & ~_PAGE_CACHE_CTL) | \
 				            _PAGE_COHERENT | _PAGE_WRITETHRU))
 
+#define pgprot_writecombine pgprot_noncached_wc
 
 struct file;
 extern pgprot_t phys_mem_access_prot(struct file *file, unsigned long pfn,