clk: ti: add mux clock driver

The driver manages a register-mapped multiplexer with multiple input
clock signals or parents, one of which can be selected as output. It
uses routines provided by the common clock framework (ccf).

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

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