ARM: dts: stm32: Repair PMIC configuration on AV96

The core and vdd PMIC buck regulators were misconfigured, which caused
instability of the board and malfunction of high-speed interfaces, like
the RGMII. Configure the PMIC correctly to repair these problems. Also,
model the missing Enpirion EP53A8LQI on the DHCOR SoM as a fixed regulator.

Reviewed-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Cc: Patrick Delaunay <patrick.delaunay@st.com>
Cc: Patrice Chotard <patrice.chotard@st.com>