asus-nb-wmi: add some video toggle keys

There are some new video switch keys that used by newer machines.
0xA0 - SDSP HDMI only
0xA1 - SDSP LCD + HDMI
0xA2 - SDSP CRT + HDMI
0xA3 - SDSP TV + HDMI
But in Linux, there is no suitable userspace application to handle this,
so, mapping them all to KEY_SWITCHVIDEOMODE.

Signed-off-by: AceLan Kao <acelan.kao@canonical.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>