regulator: fix: enable gpio when requested

The fix in commit b7adcdd0 has the side-effect that the regulator
will be disabled when requesting the relevant gpio in
regulator_common_ofdata_to_platdata() and enabled in
regulator_pre_probe() when the regulator was already enabled.
This leads to a short interruption in the 3.3V power to the PCIe
slot on the firefly-rk3399 which makes an ADATA SX8000NP NVMe SSD
unhappy.

Fix this by setting the GPIOD_IS_OUT_ACTIVE flag again when the
'regulator-boot-on' property is set, but check for this property
explicitly instead of relying on the "boot_on" member of
the uclass platdata.

Signed-off-by: Mark Kettenis <kettenis@openbsd.org>
Tested-by: Patrice Chotard <patrice.chotard@st.com>