dts/bindings: add '#cells' to nrf-saadc

This is to support e.g. "<&adc 3>" in the device tree to create e.g.

  DT_FOO_IO_CHANNELS_CONTROLLER = "ADC_0"
  DT_FOO_IO_CHANNELS_INPUT = 3

Signed-off-by: Jim Paris <jim@jtan.com>