ACPI: thinkpad-acpi: clean up probing and move init to subdrivers

Move most of the probing code to its own function, and most of the
subdriver-specific init code into subdriver init functions.

This allows us to not define pci_handle unless the dock subdriver is
enabled, as well.

This patch causes a minor userland interface change: if a subdriver doesn't
detect a capability, /proc entries for it are not created anymore (as
opposed to a /proc entry that just returned "unsupported").

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