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/nic3com.c b/nic3com.c
index eec24bf..fbd5e0d 100644
--- a/nic3com.c
+++ b/nic3com.c
@@ -117,7 +117,7 @@
 	 */
 	OUTW(SELECT_REG_WINDOW + 0, io_base_addr + INT_STATUS);
 
-	return register_par_master(&par_master_nic3com, BUS_PARALLEL, 128*KiB, NULL);
+	return register_par_master(&par_master_nic3com, BUS_PARALLEL, 0, 128*KiB, NULL);
 }
 
 static void nic3com_chip_writeb(const struct flashctx *flash, uint8_t val,