PPC 85xx: Add ELF entry point

We want to be able to directly execute the ELF binary without going
through the u-boot.bin one.

To know where we have to start executing this ELF binary  we have to
tell the linker where our entry point is.

Signed-off-by: Alexander Graf <agraf@suse.de>
Acked-by: Scott Wood <scottwood@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>