Input: elants - document some registers and values

Add information found in downstream kernels, to make the code less
magic.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Reviewed-by: Dmitry Osipenko <digetx@gmail.com>
Tested-by: Dmitry Osipenko <digetx@gmail.com>
Link: https://lore.kernel.org/r/728fff020bc92be10d84cc2a7ea8af6fd99af96c.1607669375.git.mirq-linux@rere.qmqm.pl


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>