powerpc/85xx: Declare fsl_ddr_set_memctl_regs in <asm/fsl_ddr_sdram.h>

Remove declerations of fsl_ddr_set_memctl_regs in board files with and
place it into a common header.

Based on patch from Poonam Aggrwal.

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