Input: psmouse - add support for Cortron PS/2 Trackballs

Cortron PS/2 Trackballs (700-0001A) report the 4th button using the 4th
bit of the first packet (yes, it breaks the standard PS/2 protocol).
This patch adds an extra protocol to generate BTN_SIDE based on the 4th
bit. There's no way to detect those trackballs using any kind of special
sequence, thus the protocol must be activated explicitely by writing
into 'protocol' sysfs attribute:

	echo -n "cortps" > /sys/devices/platform/i8042/serio1/protocol

Signed-off-by: Aristeu Rozanski <arozansk@redhat.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>