cmd: pxe: Add relocation to pxe_sub cmds

pxe sub commands need to be manually relocated for architectures which
enables MANUAL_RELOC as Microblaze.

Signed-off-by: T Karthik Reddy <t.karthik.reddy@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>