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>