stm32mp1: manage regulator-always-on property

If the regulator-always-on property is set for one of the regulators,
then the always_on field of the regu struct is set to true.

Change-Id: Ia6231bd187cd5ee8436bd57f810a29903129a474
Signed-off-by: Yann Gautier <yann.gautier@st.com>
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/151934


Tested-by: Antonio Maria BORNEO <antonio.borneo@st.com>
Reviewed-by: CITOOLS <smet-aci-reviews@lists.codex.cro.st.com>
Reviewed-by: Antonio Maria BORNEO <antonio.borneo@st.com>
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/152230


Reviewed-by: CIBUILD <smet-aci-builds@lists.codex.cro.st.com>