toshiba_acpi: Add support for USB Sleep functions under battery

Toshiba laptops supporting USB Sleep and Charge also come with a
feature called "USB functions under battery", which what it does when
enabled, is allows the USB Sleep functions when the computer is under
battery power.

This patch adds support to that function, creating a sysfs entry
named "sleep_functions_on_battery", accepting values from 0-100,
where zero disables the function and 1-100 sets the battery level at
which point the USB Sleep functions will be disabled, and printing
the current state of the functon and also the battery level currently
set.

Signed-off-by: Azael Avalos <coproscefalo@gmail.com>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>