stm32mp1: add plat_scmi_clock_set_rate()

Implement dedicated platform function plat_scmi_clock_set_rate(),
to override the weak defined function.

Change-Id: I651e3060588196baa52cf8d80f1ad9af528cf142
Signed-off-by: Gabriel Fernandez <gabriel.fernandez@st.com>
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/185719


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>