pinctrl: mtmips: add support for MediaTek MT7621 SoC

This patch adds pinctrl support for MediaTek MT7621 SoC.
The MT7621 SoC supports pinconf, but it is not the same as mt7628.

Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>