pinctrl: add support for MediaTek MT7628

This patch adds pinctrl support for mt7628, with a file for common pinmux
functions and a file for mt7628 which has additional support for pinconf.

diff --git a/drivers/pinctrl/Makefile b/drivers/pinctrl/Makefile
index 4b080b7..ce0879a 100644
--- a/drivers/pinctrl/Makefile
+++ b/drivers/pinctrl/Makefile
@@ -9,6 +9,7 @@
 obj-$(CONFIG_$(SPL_)PINCTRL_ROCKCHIP)	+= rockchip/
 obj-$(CONFIG_ARCH_ASPEED) += aspeed/
 obj-$(CONFIG_ARCH_ATH79) += ath79/
+obj-$(CONFIG_ARCH_MTMIPS) += mtmips/
 obj-$(CONFIG_ARCH_RMOBILE) += renesas/
 obj-$(CONFIG_PINCTRL_SANDBOX)	+= pinctrl-sandbox.o