Handle Asynchronous DDR clock on 85xx

The MPC8572 introduces the concept of an asynchronous DDR clock with
regards to the platform clock.

Introduce get_ddr_freq() to report the DDR freq regardless of sync/async
mode.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
diff --git a/cpu/mpc85xx/spd_sdram.c b/cpu/mpc85xx/spd_sdram.c
index 2a4cd57..553f736 100644
--- a/cpu/mpc85xx/spd_sdram.c
+++ b/cpu/mpc85xx/spd_sdram.c
@@ -53,8 +53,8 @@
 {
 	int clks;
 
-	clks = picos / (2000000000 / (get_bus_freq(0) / 1000));
-	if (picos % (2000000000 / (get_bus_freq(0) / 1000)) != 0) {
+	clks = picos / (2000000000 / (get_ddr_freq(0) / 1000));
+	if (picos % (2000000000 / (get_ddr_freq(0) / 1000)) != 0) {
 		clks++;
 	}
 
@@ -421,7 +421,7 @@
 	 * Adjust the CAS Latency to allow for bus speeds that
 	 * are slower than the DDR module.
 	 */
-	busfreq = get_bus_freq(0) / 1000000;	/* MHz */
+	busfreq = get_ddr_freq(0) / 1000000;	/* MHz */
 
 	effective_data_rate = max_data_rate;
 	if (busfreq < 90) {