clk: st: STiH407: Support for Flexgen Clocks

This patch is the Flexgen implementation reusing as much as possible
of Common Clock Framework functions.

The idea is to have an instance of "struct flexgen" per output clock.
It represents the clock cross bar (by a mux element), and the pre and final dividers
(using dividers and gates elements).

Signed-off-by: Maxime Coquelin <maxime.coquelin@st.com>
Signed-off-by: Olivier Bideau <olivier.bideau@st.com>
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>