tests: drivers: regulator: add support for RT685 to voltage test

Add support for testing the onboard PCA9420 regulator on the RT685 EVK
to the regulator voltage test. Since the ADC can only record voltages up
to 1.8V, and some regulators cannot have all voltage ranges tested
without crashing the system, only LDO1 and LDO2 are tested in this
testsuite

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