clk: ti: add gate clock driver

The patch adds support for TI gate clock binding. The code is based on
the drivers/clk/ti/gate.c driver of the Linux kernel version 5.9-rc7.
For DT binding details see:
- Documentation/devicetree/bindings/clock/ti/gate.txt

Signed-off-by: Dario Binacchi <dariobin@libero.it>