iio: imu: adis16480: Add support for configurable drdy indicator

The FNCTIO_CTRL register provides configuration control for each I/O pin
(DIO1, DIO2, DIO3 and DIO4).

This patch adds the option to configure each DIOx pin as data ready
indicator with positive or negative polarity by reading the 'interrupts'
and 'interrupt-names' properties from the devicetree. The
'interrupt-names' property is optional, if it is not specified, then the
DIO1 pin is used as default data ready signal.

Although the factory default assigns DIO2 as data ready signal, in the
versions previous this patch, DIO1 pin was used. We should leave this
configuration as is, since some devices might be expecting the interrupt
on the wrong physical pin.

Signed-off-by: Stefan Popa <stefan.popa@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>