ACPI: button: use .notify method instead of installing handler directly

This patch adds a .notify() method.  The presence of .notify() causes
Linux/ACPI to manage event handlers and notify handlers on our behalf,
so we don't have to install and remove them ourselves.

Note that events from fixed hardware buttons now show up as a special
notify event, so to preserve user-space backward compatibility, we
convert that back to ACPI_BUTTON_NOTIFY_STATUS.

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Reviewed-by: Alex Chiang <achiang@hp.com>
CC: Alexey Starikovskiy <alexey.y.starikovskiy@linux.intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>