86xx: Use gc-sections to reduce image size

On an XPedite5170 over 11KBytes were saved:
  Before:
     text	   data	    bss	    dec	    hex	filename
   319488	  28700	  33204	 381392	  5d1d0	./u-boot

  After:
     text	   data	    bss	    dec	    hex	filename
   307663	  29144	  33204	 370011	  5a55b	./u-boot

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>