drivers: adc: support side B channels in LPADC driver

Add support for side B channels in MCUX LPADC driver. Given
that no instances of the IP block have more than 8 a side channels,
use channel numbers over 8 to indicate side B channel is desired.

Fixes #51076

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>