ARM: dts: stm32: fix a typo for DAC io-channel-cells on stm32f429

Fix a typo on STM32F429 DAC, e.g. s/channels/channel

Fixes: 25329b23 ("ARM: dts: stm32: Add DAC support on stm32f429")

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