ACPI: thinkpad-acpi: do not use named sysfs groups

The initial version of the thinkpad-acpi sysfs interface (not yet released
in any stable mainline kernel) made liberal use of named sysfs groups, in
order to get the attributes more organized.

This proved to be a really bad design decision.  Maybe if attribute groups
were as flexible as a real directory, and if binary attributes were not
second-class citizens, the idea of subdirs and named groups would not have
been so bad.

This patch makes all the thinkpad-acpi sysfs groups anonymous (thus
removing the subdirs), adds the former group names as a prefix (so that
hotkey/enable becomes hotkey_enable for example), and updates the
documentation.

These changes will make the thinkpad-acpi sysfs ABI a lot easier to
maintain.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Len Brown <len.brown@intel.com>