dt-bindings: HID: i2c-hid: elan: Introduce bindings for Elan eKTH6915

Like many i2c-hid touchscreen controllers, the Elan eKTH6915
controller has a reset gpio. For the Goodix GT7375P touchscreen the
decision was to add a new binding rather than trying to add a new GPIO
to the existing i2c-hid binding. We'll follow the lead and do it here,
too.

SIDE NOTE: the Elan eKTH6915 is a touchscreen _controller_ that's
included as a part on some touchscreens. The reset line isn't truly
necessary for the functioning of the touchscreen, so it's possible
that some designs won't have it hooked up and will just guarantee the
power sequencing requirements with RLC circuits. Thus, we'll mark the
reset gpio as optional.

Note that if the reset GPIO isn't used there's actually no true need
to use the "elan,ekth6915" compatible instead of the "hid-over-i2c" on
Linux. However:
- Officially using just "hid-over-i2c" for this device violates the
  existing "hid-over-i2c" bindings. The bindings say that you're not
  supposed to use "post-power-on-delay-ms" without specifying a more
  specific compatible. Currently the Linux driver doesn't enforce
  this, but it violates the bindings to just use
  "hid-over-i2c". ...and if you're going to add a more specific
  compatible anyway, might as well do it right.
- Using this compatible means we don't need to specify
  "hid-descr-addr" since it's inferred from the compatible.
- Using this compatible means that the regulator names match the names
  on the Elan datasheet (vcc33 / vccio) vs the generic hid-over-i2c
  (vdd / vddl).

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>