drivers: adc: rewrite mcux LPADC driver

Rewrite MCUX LPADC driver, to better utilize hardware.
the following changes have been applied:
- channel numbers now correspond to hardware channel command slots,
  use "input_positive" and "input_negative" fields along with channel
  definitions in dt-bindings/adc/mcux-lpadc.h to select a channel
- the number of channel command slots available is configurable via
  CONFIG_LPADC_CHANNEL_COUNT
- Side A and side B channels are now supported
- differential channel mode is now supported
- ADC channels now are sampled via hardware, without additional
  software triggering

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