power: regulator: stm32: vrefbuf: fix a possible overshoot when re-enabling

There maybe an overshoot:
- when disabling, then re-enabling vrefbuf too quickly
- or upon platform reset as external capacitor maybe slow
  discharging (VREFBUF is HiZ at reset by default).
VREFBUF is used by ADC/DAC on some boards. An overshoot on the reference
voltage make the conversions inaccurate for a short period of time. So:
- Don't put the VREFBUF in HiZ when disabling, to force an active
  discharge.
- Enforce a 1ms OFF/ON delay, also upon reset

Penalty is a 1ms delay is applied (even for a cold boot), when enabling
VREFBUF.

Fixes: 93cf0ae7 ("power: regulator: Add support for stm32-vrefbuf")

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>