Input: gpio-keys - add support for disabling gpios through sysfs

Now gpio-keys input driver exports 4 new attributes to userland through
sysfs:
	/sys/devices/platform/gpio-keys/keys [ro]
	/sys/devices/platform/gpio-keys/switches [ro]
	/sys/devices/platform/gpio-keys/disabled_keys [rw]
	/sys/devices/platform/gpio-keys/disables_switches [rw]

With these attributes, userland program can read which keys and
switches can be disabled and then disable/enable them as needed.
Keys and switches are exported as stringified bitmap of codes
(keycodes or switch codes). For example keys 15, 89, 100, 101,
102 are exported as: '15,89,100-102'.

Description of the attributes:
	keys - bitmap of keys which can be disabled
	switches - bitmap of switches which can be disabled
	disabled_keys - bitmap of currently disabled keys
			(bit 1 means disabled, 0 enabled)
	disabled_switches - bitmap of currently disabled switches
			(bit 1 means disabled, 0 enabled)

Signed-off-by: Mika Westerberg <ext-mika.1.westerberg@nokia.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>