samples: drivers: adc: Update acquisition time property configuration

Enabled nxp lpadc driver acquisition time feature, the overlay file
can express acquisition time in microseconds, nanosecond and ADC
ticks.

The purpose of updating these overlay files is not only for local
testing, but also to remind users that they can use this feature.

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