Anders Grafström reports that footbridge fails to build after 1c4a4f48.
Fix this by adding the necessary definitions for __pfn_to_bus and
__bus_to_pfn.
Reported-by: NAnders Grafström <anders.grafstrom@netinsight.net>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>