Fix build breakage on platforms, not providing readsw and writesw
functions, e.g., on x86(_64).
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Chris Ball <cjb@laptop.org>