RFC small change to tsl2563 event interface.

[Jonathan Cameron <jic23@cam.ac.uk>]

Hi Amit,

As part of moving to an registration of IIO channels through
a table of iio_chan_spec structures, I'm proposing to make
a small change to the event interface for the tsl2563 driver.

Currently it has 

 intensity0_both_thresh_en as a single sysfs attr enabling both
rising and falling attributes (which have separate value attributes).

I propose changing this to a pair of attrs which are interlinked
(e.g. change one and you'll change the other - our interface
spec has always allowed this as it's vital in more complex
interlinked event cases).

intensity0_both_thresh_falling_en     intensity0_both_thresh_rising_en

It removes a fiddly special case (which I haven't figured out how to
do!) from the core code.

Any objections to this?

Thanks,

Jonathan