clk: st: Support for A9 MUX clocks

The patch supports the A9-mux clocks used by ClockGenA9

A9-mux clock : Multiplexer inside ClockGenA9. A9 clock can be driven by
either PLL or External clock (with an optional divide-by-2). This is
implemented as 3-parent clock : PLL, Ext-clk OR Ext-clk/2

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