pmic: stm32mp_pmic: add supply services

dt_pmic_find_supply() finds in the FDT the supply name related to a
regulator name.
pmic_set_regulator_min_voltage() sets target supply to its min voltage
specified in the FDT by property "regulator-min-microvolt".

Change-Id: Ic5fcabb7b2353deb1af20b7cc76e5bceca70bc77
Signed-off-by: Nicolas Le Bayon <nicolas.le.bayon@st.com>
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/160673


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>