hexagon: add memset_io() helper

We already have memcpy_toio(), but not memset_io(), so let's
add the obvious version to allow building an allmodconfig kernel
without errors like

drivers/gpu/drm/ttm/ttm_bo_util.c: In function 'ttm_bo_move_memcpy':
drivers/gpu/drm/ttm/ttm_bo_util.c:390:3: error: implicit declaration of function 'memset_io' [-Werror=implicit-function-declaration]
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Richard Kuo <rkuo@codeaurora.org>

arch/hexagon/include/asm/io.h

@@ -216,6 +216,12 @@ static inline void memcpy_toio(volatile void __iomem *dst, const void *src,
 	memcpy((void *) dst, src, count);
 }
 
+static inline void memset_io(volatile void __iomem *addr, int value,
+			     size_t size)
+{
+	memset((void __force *)addr, value, size);
+}
+
 #define PCI_IO_ADDR	(volatile void __iomem *)
 
 /*