dell-wmi: Add keys for Dell XPS L502X

All of these keys are being reported on the keyboard
controller but are also generating WMI events. Add them
to the legacy keymap to silence the noise.

BugLink: http://bugs.launchpad.net/bugs/815914


Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>