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>