Fix memory initialization on MPC8349E-mITX

Define CFG_DDR_SDRAM_CLK_CNTL for the MPC8349E-mITX and MPC8349E-mITX-GP.
This allows ddr->sdram_clk_cntl to be properly initialized.  This is necessary
on some ITX boards, notably those with a revision 3.1 CPU.

Also change spd_sdram() in cpu/mpc83xx/spd_sdram.c to not write anything into
ddr->sdram_clk_cntl if CFG_DDR_SDRAM_CLK_CNTL is not defined.

Signed-off-by: Timur Tabi <timur@freescale.com>
Acked-by: Michael Benedict <MBenedict@twacs.com>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>