memory_bus: Move (un)map_flash_region into par master

Now that the map/unmap_flash functions are only called from memory-
mapped chip drivers, we can safely move the hooks into the parallel
masters.

This also allows us to move the code away from the globals in
`flashprog.c` into a new `memory_bus.c`.

Change-Id: Ic476cf4d96200232900537b997e1d07bb4e8b809
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.sourcearcade.org/c/flashprog/+/72522
Reviewed-by: Riku Viitanen <riku.viitanen@protonmail.com>
12 files changed