dell-wmi: Support new hotkeys on the XPS 13 9350 (Skylake)

The XPS 13 9350 sends WMI keypress events that aren't enumerated in
the DMI table.  Add a table listing them.  To avoid breaking things
that worked before, these un-enumerated hotkeys won't be used if the
DMI table maps them to something else.

FWIW, it appears that the DMI table may be a legacy thing and we
might want to rethink how we handle events in general.  As an
example, a whole lot of things map to KEY_PROG3 via the DMI table.

This doesn't send keypress events for any of the new events.  They
appear to all be handled by other means (keyboard illumination is
handled automatically and rfkill is handled by intel-hid).

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Acked-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>