driver: regulator: Improve nxp vref driver.

1.Remove the code that sets the UTRIM register to 0,
because UTRIM is automatically loaded with a factory-trimmed
value, zeroing will affect the accuracy. Only need to clear
VREF UTRIM[TRIM2V1] at the initialization stage.

2. VREF does not have NXP_VREF_MODE_INTERNAL_REGULATOR mode,
the internal voltage regulator and chop oscillator are used
to suppress power supply noise and reduce voltage offset,
so remove the NXP_VREF_MODE_INTERNAL_REGULATOR mode.
there are only three modes in VREF, the first is the standby
mode (CSR[BUF21EN] = 0, CSR[HI_PWR_LV] = X), the second is
low power mode (CSR[BUF21EN] = 1, CSR[HI_PWR_LV] = 0), and
the third is high power mode (CSR[BUF21EN] = 1, CSR[HI_PWR_LV] = 1).

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