pinctrl: aspeed: Add SIG_DESC_CLEAR() helper

The complement of SIG_DESC_SET().
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://lore.kernel.org/r/20190729055604.13239-6-andrew@aj.id.auSigned-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/pinctrl/aspeed/pinmux-aspeed.h

@@ -508,6 +508,7 @@ struct aspeed_pin_desc {
  * @idx: The bit index in the register
  */
 #define SIG_DESC_SET(reg, idx) SIG_DESC_IP_BIT(ASPEED_IP_SCU, reg, idx, 1)
+#define SIG_DESC_CLEAR(reg, idx) SIG_DESC_IP_BIT(ASPEED_IP_SCU, reg, idx, 0)
 
 #define SIG_DESC_LIST_SYM(sig, group) sig_descs_ ## sig ## _ ## group
 #define SIG_DESC_LIST_DECL(sig, group, ...) \