MX31: mx31pdk: fix nand_spl

Commit f326cbba98 breaks mx31pdk,
as the _end section in u-boot.lds is missing for the nand_spl
production.

Signed-off-by: Stefano Babic <sbabic@denx.de>
CC: Fabio Estevam <fabio.estevam@freescale.com>
Tested-by: Fabio Estevam <fabio.estevam@freescale.com>

nand_spl/board/freescale/mx31pdk/u-boot.lds

@@ -63,6 +63,8 @@ SECTIONS
 		*(.dynsym)
 	}
 
+	_end = .;
+
 	.bss __rel_dyn_start (OVERLAY) : {
 		__bss_start = .;
 		*(.bss)