powerpc/mpc8536DS:Increase binary size for mpc8536DS board

u-boot binary size for Freescale mpc8536DS platforms is 512KB.
This has been reached to upper limit of the platforms and causig
linker error. So increase the u-boot binary size to 768KB.

Signed-off-by: Haijun Zhang <Haijun.Zhang@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>