Fix a typo on STM32MP15 DAC, e.g. s/channels/channel

Fixes: da6cddc7 ("ARM: dts: stm32: Add DAC support to stm32mp157c")
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>