thinkpad_acpi: Add adaptive_kbd_mode sysfs attr

Add a sysfs attribute to allow privileged users to change the keyboard
mode. This could be used by desktop environments to change the keyboard
mode depending on the application focused, as the Windows application
does.

Signed-off-by: Bastien Nocera <hadess@hadess.net>
Acked-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>