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/atavia.c b/atavia.c
index c788b1b..bc70268 100644
--- a/atavia.c
+++ b/atavia.c
@@ -159,7 +159,7 @@
 		return 1;
 	}
 
-	return register_par_master(&lpc_master_atavia, BUS_LPC, 0, NULL);
+	return register_par_master(&lpc_master_atavia, BUS_LPC, 0, 0, NULL);
 }
 
 static void atavia_chip_writeb(const struct flashctx *flash, uint8_t val, const chipaddr addr)