Add abs-rel mode and BTN_TOUCH notification.

[geeksville]

Hi,

Thanks for making this slick driver. I just used it on my recently purchased Beekeeb Toucan. One issue I noticed is that I wanted a way to get touch notifications. With touch notification I was able to add a layer that instantly turns on/off anytime a finger is touching the trackpad. This allows adding an input processor to switch layers to a layer that remaps some keys to be mouse buttons.

As I'm sure you know - to get touch data (z height) from the pinnacle part the chip needs to be in abs mode. So when this mode is enabled via the device-tree I use the @halfdane existing abs work (included as a dependency for my change) to configure the part. But I do my own math to convert the abs coords into relative moves before forwarding up the stack.

I don't know if you want this as a PR, but if you do here it is. If you have interest I'm happily willing to update the PR to match whatever requirements your project has.

Rock on.

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This mode operates the chip in absolute mode (with all of the existing
absolute options).  But reports events as RELATIVE.  This allows use as a
relative touchpad but the addition of EVENT_BTN_TOUCH notifications when
a finger is either on or off of the touchpad.

Such notifications are especially useful to automate instantly switching to/from
a layer that has keys bounds as 'mouse buttons'. (with none of the lag/delays of
'temporary layers' or the cost of constantly sending bluetooth packets between
two halves of a split keybaord)

[geeksville]

for example usage of this mode see: beekeeb/zmk-keyboard-toucan#7