platform/x86: dell-wmi: Add events created by Dell Rugged 2-in-1s

The Dell Rugged 7202 has 3 programmable buttons (labeled P1, P2, P3)
and a detachable keyboard/mouse dock.

Signed-off-by: Mario Limonciello <mario_limonciello@dell.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>