uboot-imx

fsl-ddr: Fix power-down timing settings

1. TIMING_CFG_0[ACT_PD_EXIT] was set to 6 clocks, but
   It should be set to tXP parameter, tXP=max(3CK, 7.5ns)
2. TIMING_CFG_0[PRE_PD_EXIT] was set to 6 clocks, but
   It should be set to tXP (if MR0[A12]=1) else to tXPDLL parameter
   We are setting the mode register MR0[A12]='1'

Signed-off-by: Dave Liu <daveliu@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>