i2c_hid: driver for i2c-hid devices

This is getting more common in tablets and laptops. It replaces PS/2 for
the internal keyboard and pointing devices. This is simpler and cheaper
than using up USB ports, and also simpler than the old and quirky PS/2
protocol.

The HID spec is the same no matter what transport is used (it is also
applicable for Bluetooth). Ideally we could create a separate HID bus
manager that would handle all these devices in a generic way, but that
is a lot of work nad extra complications for uncertain gains.

For now, just move the common files to a shared directory where both
drivers can use them. As a result the files are compiled twice, which is
what we want, because currently they hardcode some device paths that
need to be different for each driver.

Change-Id: I0327f6864dd0a4372b708f7b7ecf299aa86a6ea9
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2466
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>