mb/gigabyte/ga-{h61m-s2pv,b75m-d3h,b75m-d3v}: Clean up mainboard code

I ported ga-h61m-s2pv based on the two Gigabyte b75m boards.
Based on another mainboard's code review comments, this patch
improves the code quality of these three similar boards.

ga-h61m-s2pv is tested and confirmed to be working, but I cannot
say the same regarding the other two mainboards as I do not have them.

Change-Id: Ib7747cceb5ba56f791677204cdc4c54c129c70c3
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/27211
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>
diff --git a/src/mainboard/gigabyte/ga-h61m-s2pv/romstage.c b/src/mainboard/gigabyte/ga-h61m-s2pv/romstage.c
index 3e63721..f1cd176 100644
--- a/src/mainboard/gigabyte/ga-h61m-s2pv/romstage.c
+++ b/src/mainboard/gigabyte/ga-h61m-s2pv/romstage.c
@@ -57,11 +57,11 @@
 
 void mainboard_config_superio(void)
 {
-	/* Enable serial port and flip some magic bits */
-
+	/* Enable serial port */
 	ite_enable_serial(SERIAL_DEV, CONFIG_TTYS0_BASE);
 
-	ite_reg_write(SUPERIO_GPIO, 0xEF, 0x7E); // magic
+	/* Disable SIO WDT which kicks in DualBIOS */
+	ite_reg_write(SUPERIO_GPIO, 0xEF, 0x7E);
 }
 
 void mainboard_get_spd(spd_raw_data *spd, bool id_only)