pxe: Ensure all memory access is to mapped memory

Properly map memory through map_sysmem so that pxe can be used from the
sandbox.

Tested in sandbox as well as on jetson-tk1, odroid-xu3, snow as peach-pi
boards

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Acked-by: Simon Glass <sjg@chromium.org>