adc-mcux: add support for adc16 channel multiplex

Channels 4..7 are multiplexed on adc16. The NXP HAL exposes
ADC16_SetChannelMuxMode but this function is not yet included
in Zephy. The following patch adds channel-mux-b to the dts
enabling the use of the alternate channels of 16 bit adc.

Signed-off-by: Andreas Dröscher <github@anticat.ch>