Print output filename as part of warning messages

For example:
src/arm/at91-ariag25.dtb: Warning (unit_address_vs_reg): Node /memory has a reg or ranges property, but no unit name

If output is to stdout then the prefix is "<stdout>: ".

This helps to direct the developer to where to look when multiple files are
being compiled in parallel.

Signed-off-by: Ian Campbell <ijc@hellion.org.uk>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

dtc.h

@@ -246,6 +246,7 @@ struct dt_info {
 	struct reserve_info *reservelist;
 	uint32_t boot_cpuid_phys;
 	struct node *dt;		/* the device tree */
+	const char *outname;		/* filename being written to, "-" for stdout */
 };
 
 /* DTS version flags definitions */