adc: rockchip-saradc: add support for getting reference voltage value

Mirroring commit 97ab802a ("adc: meson-saradc: add support for
getting reference voltage value") for meson-saradc, this adds support
for getting the "vref-supply" regulator and register it as the ADC's
reference voltage regulator, so clients can translate sampled ADC values
to voltage.

Signed-off-by: Peter Cai <peter@typeblog.net>
Reviewed-by: John Keeping <john@metanate.com>
Tested-by: John Keeping <john@metanate.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Philipp Tomsich <philipp.tomsich@vrull.eu>
Cc: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>