asus-laptop: add Lenovo SL hotkey support

Lenovo SL series laptop has a very similar DSDT with Asus laptops. We can
easily have the extra ACPI function support with little modification in
asus-laptop.c

Here is the hotkey enablement for Lenovo SL series laptop.

This patch will enable the following hotkey:
 - Volumn Up
 - Volumn Down
 - Mute
 - Screen Lock (Fn+F2)
 - Battery Status (Fn+F3)
 - WLAN switch (Fn+F5)
 - Video output switch (Fn+F7)
 - Touchpad switch (Fn+F8)
 - Screen Magnifier (Fn+Space)

The following function of Lenovo SL laptop is still need to be enabled:
 - Hotkey: KEY_SUSPEND (Fn+F4), KEY_SLEEP (Fn+F12), Dock Eject (Fn+F9)
 - Rfkill for bluetooth and wlan
 - LenovoCare LED
 - Hwmon for fan speed
 - Fingerprint scanner
 - Active Protection System

Signed-off-by: Ike Panhc <ike.pan@canonical.com>
Signed-off-by: Len Brown <len.brown@intel.com>