stm32mp: use newly introduced minimal clock framework

Change call of:
stm32mp_clk_enable() / stm32mp_clk_disable() / stm32mp_clk_get_rate()
by
clk_enable() / clk_disable() / clk_get_rate()

Change-Id: I15d2ce57b9499211fa522a1b53eeee9cf584c111
Signed-off-by: Gabriel Fernandez <gabriel.fernandez@st.com>
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/182346


Reviewed-by: CITOOLS <smet-aci-reviews@lists.codex.cro.st.com>
Reviewed-by: Yann GAUTIER <yann.gautier@st.com>
Tested-by: Yann GAUTIER <yann.gautier@st.com>