dell-rbtn: Dell Airplane Mode Switch driver

This is an ACPI driver for Dell laptops which receive HW slider radio
switch or hotkey toggle wifi button events. It exports rfkill device
dell-rbtn (which provide correct hard rfkill state) or hotkey input device.

Alex Hung is author of original hotkey input device code.

Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
Tested-by: Gabriele Mazzotta <gabriele.mzt@gmail.com>
Cc: Alex Hung <alex.hung@canonical.com>
[fengguang.wu@intel.com: rbtn_ops can be static]
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
[dvhart@linux.intel.com: Correct multi-line comment formatting]
Signed-off-by: Darren Hart <dvhart@linux.intel.com>