drivers/ddr/fsl: Adjust bstopre value

By default the bstopre value has been set to 0x100, used to be 1/4
value of refint. Modern DDR has increased the refresh time. Adjust
to 1/4 of refresh interval dynamically. Individual board can still
override this value in board ddr file, or to use auto-precharge.

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