ARM64: dts: rk3399: add trackpad for gru/kevin boards

ARM64: dts: rk3399: add trackpad for gru/kevin boards

The trackpad bits in the DTS needed some love.  This adds some basic
infrastructure support in the main gru dts file and then adds the
specific trackpad used on kevin-r0 and kevin-r1.  For now just duplicate
between kevin-r0 and kevin-r1 and we'll decide if we want to share
later (perhaps we want an "atmel" snippet?).

Note that gpio-keymap here makes the driver appear as a trackpad rather
than a touchscreen (driver assumes that anything with buttons is a
trackpad).  Input entry corresponding to the button on the trackpad was
found by experimentation as suggested in the device tree bindings.

BUG=chrome-os-partner:52637
TEST=With series, trackpad works in browser; button works.

Change-Id: Ia62cff90449625778fd99054b914e22a55c13550
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-on: https://chrome-internal-review.googlesource.com/256510
Commit-Ready: Doug Anderson <dianders@google.com>
Tested-by: Doug Anderson <dianders@google.com>
Reviewed-by: Brian Norris <briannorris@google.com>
Signed-off-by: Caesar Wang <wxt@rock-chips.com>