util/bincfg: Add MAC address example to gbe-ich9m.set

It's not obvious how to set specific byte of a multi-byte field in the
set file. Add an example (and a template) for setting MAC address.

Change-Id: Iea983071682ffebd61757497d43c70cc8214043d
Signed-off-by: Evgeny Zinoviev <me@ch1p.io>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/39664
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Swift Geek (Sebastian Grzywna) <swiftgeek@gmail.com>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Jacob Garber <jgarber1@ualberta.ca>

util/bincfg/gbe-ich9m.set

@@ -72,5 +72,13 @@
 	"ssdid" = 0x20ee,
 	"ssvid" = 0x17aa,
 	"did" = 0x10f5,
-	"vid" = 0x8086
+	"vid" = 0x8086,
+
+	# This example sets MAC address to 00:11:22:33:44:55
+	"macaddress0" = 0x00,
+	"macaddress1" = 0x11,
+	"macaddress2" = 0x22,
+	"macaddress3" = 0x33,
+	"macaddress4" = 0x44,
+	"macaddress5" = 0x55
 }