powerpc/85xx: Fix lds for nand build

Fix u-boot-nand.lds and u-boot-nand_spl.lds according to:

Author: Peter Tyser <ptyser@xes-inc.com>
Date:   Wed Sep 29 14:05:56 2010 -0500
commit fbe53f59
    85xx: Use gc-sections to reduce image size

Signed-off-by: Haiying Wang <Haiying.Wang@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>