stm32mp1: scmi: introduce clock rate description by step

All clocks are exposed as single rate clocks.
Concerning MPU clock, only CPU OPP can act and only valid rates
are expected for this very clock.

Change-Id: I2f76135996da23dae590aa9d77e3bfd6c03a68c1
Signed-off-by: Gabriel Fernandez <gabriel.fernandez@st.com>
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/185717


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