clk: st: Support for DIVMUX and PreDiv Clocks

The patch supports the DIVMUX and PreDiv clocks used by ClockGenA(s)

DIVMUX clock : Divider-Multiplexer-Gate inside ClockGenA(s)
It includes support for each channel : 3-parent Multiplexer,
Divider for each Parent, Gate to switch OFF the output channel. The
clock is implemented using generic clocks implemented in the kernel
clk_divider and clk_mux.

PreDiv clock : Fixed Divider Clock used inside ClockGenA(s) to divide
the oscillator clock by factor-of-16. The clock is implemented using
generic clocks implemented in the kernel clk_divider.

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>