powerpc/85xx:Fix lds for nand boot debug info

Currently "u-boot", the elf file generated via u-boot-nand.lds does not
contain required debug information i.e. .debug_{line, info, abbrev, aranges,
ranges} into their respective _global_ sections.

The original ld script line arch/powerpc/cpu/mpc85xx/start.o
KEEP(*(.bootpg)) is not entirely correct because the start.o file is already
processed by the linker,therefore the file wildcard in "KEEP(*(.bootpg))" will
not process start.o again for bootpg.

So Fix u-boot-nand.lds to generate these debug information.

Signed-off-by: Anmol Paralkar <b07584@freescale.com>
Signed-off-by: John Russo <John.Russo@freescale.com>
Signed-off-by: Prabhakar Kushwaha <prabhakar@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>