dell-laptop: support Synaptics/Alps touchpad led

This patch supports Dell laptop with Synaptics and Alps touchpad chip
that with LED to indicate the functionality of touchpad is disabled or
enabled.

The command for touchpad LED is 0x97, and the data 1 means turn on the
touchpad LED, 2 means turn it off.

BTW, I add dell_quirks to white list those machines that supports this
behavior, so that the code won't affect those who don't have a touchpad LED
machine.

We can easily to turn it on/off by
   echo 1 > /sys/class/leds/dell-laptop::touchpad/brightness
   echo 0 > /sys/class/leds/dell-laptop::touchpad/brightness

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