clk: st: Adds Flexgen clock binding

A Flexgen structure is composed by:
- a clock cross bar (represented by a mux element)
- a pre and final dividers (represented by a divider and gate elements)

Signed-off-by: Gabriel Fernandez <gabriel.fernandez@linaro.org>
Acked-by: Peter Griffin <peter.griffin@linaro.org>
Signed-off-by: Mike Turquette <mturquette@linaro.org>