RFC input: rotary_encoder: use more than two gpios as input

Hello,

some time ago I worked on the rotary encoder driver to make it support more than two input lines. This is the (only slightly tested[1]) rebase of this series on top of 4.4-rc2 (from 4.1).

It would be great to get some feedback, especially (but not only) for my change to raumfeld.c.

Before Ezequiel's patch 3a341a4c30d4 ("Input: rotary-encoder - add support for quarter-period mode") we had a dt property "rotary-encoder,half-period" defined. It's presence meant that we had 2 indents per period; it's absence that there is only 1. Ezequiel introduced rotary-encoder,steps-per-period instead when adding support for quarter-period mode (which has 4 indents per period).

Up to now (i.e. with two inputs) a period has length 4. Now with (say) four inputs a period has length 16 instead. Now how should this be modeled in dt? This series implements that I have to pass

	rotary-encoder,steps-per-period = <16>;

now for "quarter-period mode" (i.e. 4 indents per 4 state changes[2]), but that feels unnatural. I'd prefer to set a property to <1> instead, meaning the device has an indent for each state change[2]. half-period mode would be <2> and full-period mode would be <4>. But I don't have a nice name for such a property and maybe it's easier to live with steps-per-period = <16>? What do you think?

Also note that these patches are not as technically versatile as possible. With 4 (n) input lines we could detect a quick rotation where the machine's latency only allows to sample after 7 (2^(n-1)-1) state changes. Currently this is not implemented, but can be done later.

Best regards
Uwe