tests: subsys: settings: fcb: Fix failure on nrf52_pca10040

Tests failed on the target because flash area reserved
for fcb data storage was to small.
This path add dts overlay file for nrf52_pac10040 board which
increases this flash area to proper size.

Fixes #8038

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