Input: ALPS - add support for touchpads with 4-directional button

The touchpad on Acer Aspire 5720, 5520 and some other Aspire models
(signature 0x73, 0x02, 0x50) has a button that can be rocked in 4
different directions. Make the driver to generate BTN_0..BTN_3 events
in response. The Synaptics driver by default maps BTN_0 and BTN_1 to
up and down, so there should be no visible changes with the old setup
that generated BTN_FORWARD and BTN_BACK (also mapped to up and down).

Signed-off-by: Maxim Levitsky <maximlevitsky@gmail.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>