arm64: dts: imx8mm-verdin: comment about i2c level shifter

Add a note about the bootloader being expected to switch on the I2C
level shifter for the TLA2024 ADC behind this PMIC.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>