iio:adc:ad7280a: Document ABI for cell balance switches

Very minimal ABI docs. This is unusual enough that I'd expect anyone
who actually wanted to touch them to go look at the datasheet.

Signed-off-by: Jonathan Cameron <[e-mail]>
Reviewed-by: Marcelo Schmitt <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/ABI/testing/sysfs-bus-iio-adc-ad7280a

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+What:		/sys/bus/iio/devices/iio:deviceX/in_voltageY-voltageZ_balance_switch_en
+KernelVersion:	5.14
+Contact:	[e-mail]
+Description:
+		Used to enable an output for balancing cells for time
+		controlled via in_voltage_Y-voltageZ_balance_switch_timer.
+
+What:		/sys/bus/iio/devices/iio:deviceX/in_voltageY-voltageZ_balance_switch_timer
+KernelVersion:	5.14
+Contact:	[e-mail]
+Description:
+		Time in seconds for which balance switch will be turned on.
+		Multiple of 71.5 seconds.