stm32mp: add function to retrieve VDD regulator

Add function to find from device tree the proper regulator
that controls the VDD domain. Rework the dt_get_cpu_regulator_name
to use the same generic function.

Signed-off-by: Lionel Debieve <lionel.debieve@st.com>
Change-Id: I75f96d30a408f6d10182fc780bc40c3971b6095f
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/185334


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>