nb/intel/sandybridge/raminit: support calling dram_freq multiple times

The PLL will never lock if the requested frequency is already set.
As the fallback may request the same frequency again exit early
to prevent a hang.

Test system:
 * Gigabyte GA-B75M-D3H
 * Intel Pentium CPU G2130

Change-Id: I625b2956346d8c50cca84def6190c076bf99dbec
Signed-off-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-on: https://review.coreboot.org/14174
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Martin Roth <martinroth@google.com>

src/northbridge/intel/sandybridge/raminit.c

@@ -810,6 +810,17 @@ static void dram_freq(ramctr_timing * ctrl)
 		/* Frequency mulitplier.  */
 		u32 FRQ = get_FRQ(ctrl->tCK);
 
+		/* The PLL will never lock if the required frequency is
+		 * already set. Exit early to prevent a system hang.
+		 */
+		reg1 = MCHBAR32(0x5e04);
+		val2 = (u8) reg1;
+		if (val2 == FRQ) {
+			printk(BIOS_DEBUG, "MCU frequency is set at : %d MHz\n",
+			       (1000 << 8) / ctrl->tCK);
+			return;
+		}
+
 		/* Step 2 - Select frequency in the MCU */
 		reg1 = FRQ;
 		reg1 |= 0x80000000;	// set running bit