toshiba_acpi: Add touchpad enable/disable support-

Toshiba laptops have two ways of letting userspace
know the touchpad has changed state, one with a
button on top of the touchpad that simply emmits
scancodes whenever enabled/disabled, and another one
by pressing Fn-F9 (touchpad toggle) hotkey.

This patch adds support to enable/disable the touchpad
by exposing the _touchpad_ file in sysfs that simply
makes a call to a SCI register, imitating what
Toshiba provided software does on Windows.

Signed-off-by: Azael Avalos <coproscefalo@gmail.com>
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>