Makefile: need to remove generated u-boot-nand_spl.lds

On MPC85xx based NAND_SPL builds we generate a u-boot-nand_spl.lds based
on output from preprocessor.  We where never removed it.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Scott Wood <scottwood@freescale.com>