ACPI: thinkpad-acpi: claim tpacpi as an official short handle (v1.1)

Unfortunately, a lot of stuff in the kernel has size limitations, so
"thinkpad-acpi" ends up eating up too much real estate.  We were using
"tpacpi" in symbols already, but this shorthand was not visible to
userland.

Document that the driver will use tpacpi as a short hand where necessary,
and use it to name the kernel thread for NVRAM polling (now named
"ktpacpi_nvramd").

Also, register a module alias with the shorthand.  One can refer to the
module using the shorthand name.

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