clk: add clock driver for MediaTek MT7620 SoC

This patch adds a clock driver for MediaTek MT7620 SoC.
This driver provides clock gate control as well as getting clock frequency
for CPU/SYS/XTAL and some peripherals.

Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>