Add pr_fmt() macro

This macro can be overridden in source files (before including common.h)
and can be used to specify a prefix for debug and error messages. An
example of how to use this is shown below:

	#define pr_fmt(fmt) "foo: " fmt

	#include <common.h>

	...
	debug("bar");

The resulting message will read:

	foo: bar

Acked-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Simon Glass <sjg@chromium.org>