tests: voltage: Improve nxp boards hardware settings

1.The VREFO is internally connected to the ADC channel,
so can change the measurement channel to simplify the
test process.

2.Change the VREF mode to NXP_VREF_MODE_HIGH_POWER as the
'NXP_VREF_MODE_INTERNAL_REGULATOR' mode is removed.

3.Increase ADC sample time to improve ADC accuracy

4.Adjust tolerance-microvolt to a suitable value,
20000uv is a more suitable value for NXP VREF,
the actual VREF tolerance-microvolt can be smaller
than this value, but hardware differences may cause
this case to fail to run on some boards.

5.Add frdm_mcxn236 and frdm_mcxw71 to test platform.

Signed-off-by: Zhaoxiang Jin <Zhaoxiang.Jin_1@nxp.com>