Platform: OLPC: add SPI MODULE_DEVICE_TABLE

The SPI bus creates a device with the modalias of "xo1.75-ec". This
fixes XO-1.75 EC driver autoloading

Fixes: 0c3d931b ("Platform: OLPC: Add XO-1.75 EC driver")
Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>