Input: goodix - add support for capacitive home button

On some x86 tablets with a Goodix touchscreen, the Windows logo on the
front is a capacitive home button. Touching this button results in a touch
with bit 4 of the first byte set, while only the lower 4 bits (0-3) are
used to indicate the number of touches.

Report a KEY_LEFTMETA press when this happens.

Note that the hardware might support more than one button, in which
case the "id" byte of coor_data would identify the button in question.
This is not implemented as we don't have access to hardware with
multiple buttons.

Signed-off-by: Sergei A. Trusov <sergei.a.trusov@ya.ru>
Acked-by: Bastien Nocera <hadess@hadess.net>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>