iio: adc: max1027: Make it optional to use interrupts

The chip has a 'start conversion' and a 'end of conversion' pair of
pins. They can be used but this is absolutely not mandatory as regular
polling of the value is totally fine with the current internal
clocking setup. Turn the interrupts optional and do not error out if
they are not inquired in the device tree. This has the effect to
prevent triggered buffers use though.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>