clk: stm32mp1_clk: rename mpu set freq function

Rename stm32mp1_set_opp_khz to stm32mp1_set_mpu_freq
as it does not handle cpu voltage.

Change-Id: I32ecc1c0c88d01e41982a45dffe68f9f657b7d2d
Signed-off-by: Pascal Paillet <p.paillet@st.com>
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/236417


Reviewed-by: CITOOLS <MDG-smet-aci-reviews@list.st.com>
Reviewed-by: CIBUILD <MDG-smet-aci-builds@list.st.com>
Reviewed-by: Yann GAUTIER <yann.gautier@foss.st.com>