samples/drivers/soc_flash_nrf: enable nrf9160_pca10090 targets

The patch provides flash offsets appropriate for running
this sample on nrf9160_pac10090 targets.
As that sample is small one, it is done with assumption that image_1
flash partition is empty.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>