drivers: flash: add flash_simulator driver

This commit adds a flash driver implementation that writes to RAM and
exports statistics through stats.h. It can be used to simulate flash
memory for testing purposes.

Signed-off-by: Emanuele Di Santo <emdi@nordicsemi.no>
Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>
Signed-off-by: Kamil Piszczek <Kamil.Piszczek@nordicsemi.no>