dm: clk: add clk driver support for stm32h7 SoCs

dm: clk: add clk driver support for stm32h7 SoCs

This driver implements basic clock setup, only clock gating
is implemented.

This driver doesn't implement .of_match as it's binded
by MFD RCC driver.

Files include/dt-bindings/clock/stm32h7-clks.h and
doc/device-tree-bindings/clock/st,stm32h7-rcc.txt
will be available soon in a kernel tag, as all the
bindings have been acked by Rob Herring [1].

[1] http://lkml.iu.edu/hypermail/linux/kernel/1704.0/00935.html

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>