iio: mma8452: support either of the available interrupt pins

This change is important in order for everyone to be easily able to use the
driver for one of the supported accelerometer chips!

Until now, the driver blindly assumed that the INT1 interrupt line is wired
on a user's board. But these devices have 2 interrupt lines and can route
their interrupt sources to one of them. Now, if "INT2" is found and matches
i2c_client->irq, INT2 will be used.

The chip's default actually is INT2, which is why probably many boards will
have it wired and can make use of this.

Of course, this also falls back to assuming INT1, so for existing users
nothing will break. The new functionality is described in the bindings doc.

Signed-off-by: Martin Kepplinger <martin.kepplinger@theobroma-systems.com>
For the binding: Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>