ACPI: dock: add immediate_undock option

Allow the driver to be loaded with an option that will allow userspace to
control whether the laptop is ejected immediately when the user presses the
button, or only when the syfs undock file is written.

if immediate_undock == 1, then when the user presses the undock button, the
laptop will send an event to userspace to notify userspace of the undock, but
then immediately undock without waiting for userspace.  This is the current
behavior, and I set this to be the default.

if immediate_undock == 0, then when the user presses the undock button, the
laptop will send an event to userspace and do nothing.  User space can query
the "flags" sysfs entry to determine if an undock request has been made by
the user (if bit 1 is set).  User space will then need to write the undock
sysfs entry to complete the undocking process.

Signed-off-by: Kristen Carlson Accardi <kristen.c.accardi@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>