clk: stm32mp13: add stm32_mux clock management

Just to introduce management of a stm32 mux clock.

Signed-off-by: Gabriel Fernandez <gabriel.fernandez@foss.st.com>
Link: https://lore.kernel.org/r/20220516070600.7692-4-gabriel.fernandez@foss.st.com


Signed-off-by: Stephen Boyd <sboyd@kernel.org>