Add abs() macro to return absolute value

This macro is generally useful to make it available in common.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>
Acked-by: Tom Rini <trini@ti.com>
Acked-by: Mike Frysinger <vapier@gentoo.org>