drivers/flash: nRF driver uses relative addresses

So fare all nRF flash memories had flash base address at 0.
nRF flash driver was implemented in such way that it
really used absolute addresses, while convention are relative
addresses (for flash_map as well), which was not visible as start
address offset was 0.
It will become visible on nRF53 which has networking flash
with non-zero base address.

This patch switch nRF flash driver to use relative addresses for flash.
UICR absolute addressing is kept.

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