ARM: dts: stm32: Move DHCOR BUCK3 VDD 2V9 adjustment to 1V8 DTSI

The Buck3 on DHCOR is used to supply IO voltage. It can output either 3V3
in the default DHCOR configuration, or 2V9 in case of AV96 DHCOR variant
which has extra Empirion DCDC converter in front of the 1V8 IO supply, or
outright 1V8 in case of 1V8 IO DHCOR without the Empirion DCDC converter.

The 2V9 mode in case of AV96 DHCOR variant is used to reduce unnecessarily
high input voltage to the Empirion DCDC converter, so move it into matching
DTSI to stop confusing users.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Alexandre Torgue <alexandre.torgue@foss.st.com>
Cc: linux-stm32@st-md-mailman.stormreply.com
To: linux-arm-kernel@lists.infradead.org
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>