Add "silead,home-button" property to entries for tablets which have
a capacitive home button (typically a windows logo on the front).

This new property is checked for by the new capacitive home button
support in the silead touchscreen driver.
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>