mfd: dln2: Add cell for initializing DLN2 ADC

mfd: dln2: Add cell for initializing DLN2 ADC

This patch extends the DLN2 driver; adding cell for adc_dln2 module.

The original patch[1] fell through the cracks when the driver was added
so ADC has never actually been usable. That patch did not have ACPI
support which was added in v5.9, so the oldest supported version this
current patch can be backported to is 5.10.

[1] https://www.spinics.net/lists/linux-iio/msg33975.html

Cc: <stable@vger.kernel.org> # 5.10+
Signed-off-by: Jack Andersen <jackoalan@gmail.com>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20211018112541.25466-1-noralf@tronnes.org