ARM: dts: stm32: Remove buck3 regulator-always-on on AV96

ARM: dts: stm32: Remove buck3 regulator-always-on on AV96

In case the regulator-always-on is present in regulator DT node,
the regulator is always reconfigured to the voltage set in DT on
probe, even if regulator_set_value() has been called before. Drop
the property from AV96 U-Boot DT and enable the regulator manually
in code, as the board already reconfigures the Buck3 regulator in
code per PMIC NVM content instead.

Fixes: 0adf10a87b1 ("ARM: dts: stm32: Configure Buck3 voltage per PMIC NVM on Avenger96")
Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>