amd/cimx/sb800/late.c: Add comment in sb800_init()

Add a comment explaining what `abcfg_reg(0xc0, 0x01FF, 0x0F4)` does.
This is a follow-up for commit 24501cae (AMD cimx/sb800: Initially
enable all GPP ports).

Change-Id: I5ac263ee088d36a7f7a2d03c1454ed647faa7147
Signed-off-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-on: http://review.coreboot.org/12190
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

src/southbridge/amd/cimx/sb800/late.c

@@ -268,6 +268,7 @@ static void sb800_init(void *chip_info)
 	sb_config->StdHeader.CALLBACK.CalloutPtr = sb800_callout_entry;
 	sb800_cimx_config(sb_config);
 
+	/* Initially enable all GPP ports 0 to 3 */
 	abcfg_reg(0xc0, 0x01FF, 0x0F4);
 }