iio: adc: sun4i-gpadc-iio: move code used in MFD probing to new function

This moves code used in MFD probing to a new sun4i_gpadc_probe_mfd
function.

This driver was initially written for A10, A13 and A31 SoCs which
already had a DT binding for this IP, thus we needed to use an MFD to
probe the different drivers without changing the DT binding of these
SoCs.

For SoCs that will require to create a DT binding for this IP, we can
avoid using an MFD, thus we need two separate functions: one for probing
via MFD and one for probing without MFD.

This split the code specific to MFD probing in a function separated from
the driver probe function.

Signed-off-by: Quentin Schulz <quentin.schulz@free-electrons.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>