platform/x86: silead_dmi: Add support for the Onda oBook 20 Plus tablet

Add touchscreen platform data for the Onda oBook 20 Plus tablet.

Firmware for this is available here:
https://github.com/onitake/gsl-firmware/blob/master/firmware/linux/silead/gsl3676-onda-obook-20-plus.fw

Signed-off-by: Nerijus Baliūnas <nerijus@users.sourceforge.net>
Acked-by: Hans de Goede <hdegoede@redhat.com>
[andy: massaged title and wrote commit message]
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>