feat(st): use newly introduced clock framework

Replace calls to stm32mp_clk_enable() / stm32mp_clk_disable() /
stm32mp_clk_get_rate() with clk_enable() / clk_disable() /
clk_get_rate().

Change-Id: I15d2ce57b9499211fa522a1b53eeee9cf584c111
Signed-off-by: Gabriel Fernandez <gabriel.fernandez@st.com>