memory_bus: Turn flashbase into a par_master member `rom_base`

Get rid of the global `flashbase`. Treat overrides for the `rom_base`
similar to `max_rom_decode`: Gather the information in `internal_data`
and then pass it to register_par_master().

Change-Id: Ib9ed7234a849fe3550200fd602226d0036da15f0
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.sourcearcade.org/c/flashprog/+/436
diff --git a/atapromise.c b/atapromise.c
index aa83577..3d50e54 100644
--- a/atapromise.c
+++ b/atapromise.c
@@ -113,7 +113,7 @@
 		  "purpose of updating the firmware of this device (padding may necessary).\n",
 		  rom_size / 1024);
 
-	return register_par_master(&par_master_atapromise, BUS_PARALLEL, rom_size, NULL);
+	return register_par_master(&par_master_atapromise, BUS_PARALLEL, 0, rom_size, NULL);
 }
 
 static void atapromise_chip_writeb(const struct flashctx *flash, uint8_t val, chipaddr addr)