mpc85xx: workaround old binutils bug

The recent change to move the .bss outside of the image gives older
binutils (ld from eldk4.1/binutils-2.16) some headache:

ppc_85xx-ld: u-boot: Not enough room for program headers (allocated 3, need 4)
ppc_85xx-ld: final link failed: Bad value

We workaround it by being explicit about the program headers and not
assigning the .bss to a program header.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>