vc/amd/agesa/f15tn: add DDR1866_FREQUENCY to DdrMaxRateTab table

This unlocks 1866 MHz frequency for AMD boards of f15tn family.
Tested on ASUS A88XM-E with A10-6700 and Crucial BLT8G3D1869DT1TX0.

Signed-off-by: Mike Banon <mikebdp2@gmail.com>
Change-Id: I547d7e9ca89524d66ee0ee307de41699d991f9fc
Reviewed-on: https://review.coreboot.org/c/coreboot/+/40490
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/vendorcode/amd/agesa/f15tn/Proc/Mem/NB/mnmct.c

@@ -122,6 +122,7 @@ MemNSyncTargetSpeedNb (
 {
   CONST UINT16 DdrMaxRateTab[] = {
     UNSUPPORTED_DDR_FREQUENCY,
+    DDR1866_FREQUENCY,
     DDR1600_FREQUENCY,
     DDR1333_FREQUENCY,
     DDR1066_FREQUENCY,