ACPI / x86: enable touchscreen on Dell Venue Pro 7139

Permanently enable the SYNA7500 touchscreen device on the Dell
Venue Pro 7139. The DSDT hides the touchscreen ACPI device on
the 7139 in the same fashion as the 7130, and needs to
be enabled in the same way.

Signed-off-by: Tristian Celestin <tristiancelestin@fastmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>