Convert the adc-keys binding to DT schema format.

The old binding has 'label' as required, but it should never be
required given it's just a human readable description.
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220608211207.2058487-6-robh@kernel.orgSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>