clk: st: Support for VCC-mux and MUX clocks

clk: st: Support for VCC-mux and MUX clocks

The patch supports the VCC-mux and MUX clocks used by ClockGenC/F

VCC-mux clock : Divider-Multiplexer-Gate inside ClockGenC/F
It includes support for each channel : 4-parent Multiplexer, Post Divide
by 1, 2, 4 or 8, Gate to switch OFF the output channel. The clock is
implemented using generic clocks implemented in the kernel clk_divider, clk_mux,
clk_gate and clk_composite (to combine all)

MUX clock : 2-parent clock used inside ClockGenC/F. The clock is implemented
using generic clocks implemented in the kernel clk_mux.

Signed-off-by: Pankaj Dev <pankaj.dev@st.com>
Signed-off-by: Gabriel Fernandez <gabriel.fernandez@st.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>