nb/intel/ironlake: Add comment about MCH scan chains

Change-Id: I3e60cfc1fd3352b8b0c7460503179425cc593d36
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/48589
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>

src/northbridge/intel/ironlake/raminit.c

@@ -101,6 +101,15 @@ static void read128(u32 addr, u64 * out)
 	out[1] = ret.hi;
 }
 
+/*
+ * Ironlake memory I/O timings are located in scan chains, accessible
+ * through MCHBAR register groups. Each channel has a scan chain, and
+ * there's a global scan chain too. Each chain is broken into smaller
+ * sections of N bits, where N <= 32. Each section allows reading and
+ * writing a certain parameter. Each section contains N - 2 data bits
+ * and two additional bits: a Mask bit, and a Halt bit.
+ */
+
 /* OK */
 static void write_1d0(u32 val, u16 addr, int bits, int flag)
 {