iio: adc: fsl-imx25-gcq: initialize regulators as needed

iio: adc: fsl-imx25-gcq: initialize regulators as needed

The driver tries to initialize all possible regulators from the DT, then
match the external regulators with each channel and then release all unused
regulators.

We can change the logic a bit to initialize regulators only when at least
one channel needs them.

This change creates a mx25_gcq_ext_regulator_setup() function that is
called only for the external regulators. If there's already a reference to
an external regulator, the function will just exit early with no error.

This way, the driver doesn't need to keep any track of these regulators
during init.

Signed-off-by: Alexandru Ardelean <aardelean@deviqon.com>
Link: https://lore.kernel.org/r/20210625074325.9237-1-aardelean@deviqon.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>