doc: Rework PORTING.md to reflect changes to flash_area API

The commit removes requirement that each system should exactly
follow proposed layout of struct flash_area and adds information
of flash_area API being extended by getter function that access
information inside flash_area type objects, allowing systems freedom
in internal implementation of flash_area.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>