clk: st: clkgen-mux: search reg within node or parent

clk: st: clkgen-mux: search reg within node or parent

In order to avoid having duplicated addresses within the DT,
only have one unit-address per clockgen and each driver within
the clockgen should look at the parent node (overall clockgen)
to figure out the reg property.  Such behavior is already in
place in other STi platform clock drivers such as clk-flexgen
and clkgen-pll.  Keep backward compatibility by first looking
at reg within the node before looking into the parent node.

Signed-off-by: Alain Volmat <avolmat@me.com>
Link: https://lore.kernel.org/r/20211218211157.188214-3-avolmat@me.com
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>