nb/intel/sandybridge: Improve CAS freq selection

The previous code seemed weird and tried to check if its selected
value is supported three times.

This also lower the clock if a selected frequency does not result in a
supported CAS number.

Change-Id: I97244bc3940813c5a5fcbd770d71cca76d21fcae
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/19716
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>

src/include/device/dram/ddr3.h

@@ -39,6 +39,8 @@
  * These values are in 1/256 ns units.
  * @{
  */
+#define NS2MHZ_DIV256 1000 << 8
+
 #define TCK_1333MHZ     192
 #define TCK_1200MHZ     212
 #define TCK_1100MHZ     232