Add a very basic <machine/bus.h> which nows seems to be mandatory in MI

code even if a port doesn't use it.

Based on code in private email from Havard Eidnes.  Thanks Havard!

sys/arch/sbmips/include/bus.h

@@ -0,0 +1,22 @@
+/* $NetBSD: bus.h,v 1.2 2010/06/13 18:14:57 simonb Exp $ */
+
+/*
+ * A very basic <machine/bus.h>.  If/once "proper" bus.h support is needed,
+ * this can simply include:
+ *
+ *	#include <mips/bus_space.h>
+ *	#include <mips/bus_dma.h>
+ *
+ * as sys/arch/evbmips/include/bus.h does.
+ */
+
+#ifndef _SBMIPS_BUS_H_
+#define	_SBMIPS_BUS_H_
+
+typedef paddr_t	bus_addr_t;
+typedef psize_t	bus_size_t;
+
+typedef int     bus_space_tag_t;
+typedef int     bus_space_handle_t;
+
+#endif /* _SBMIPS_BUS_H_ */