ARM: dts: stm32: remove fixed regulator unit address on stm32429i-eval

vref fixed regulator shouldn't have unit address and reg properties.
Rename the label and phandle to "vref" according to the schematics.
Also remove it from simple-bus.

Fixes: 7465d811 ("ARM: dts: stm32: enable ADC on stm32f429i-eval
board")

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>