.gitmodules: Update intel-microcode submodule to track branch=main

The 3rdparty submodule 'intel-microcode' has changed the branch from
'master' to 'main'. As we do not set any specific branch name in our
config, it defaults to 'master' which makes
"git submodule update --remote --rebase 3rdparty/intel-microcode"
to fail.

This patch adds the branch name in .gitmodules to match the upstream
name.

Change-Id: I7b6d7921a21af4eb3bcc7ce4e5a8ea21c38c89a3
Signed-off-by: Werner Zeh <werner.zeh@siemens.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/55305
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Mario Scheithauer <mario.scheithauer@siemens.com>
Reviewed-by: Marc Jones <marc@marcjonesconsulting.com>

.gitmodules

@@ -34,6 +34,7 @@
 	url = ../intel-microcode.git
 	update = none
 	ignore = dirty
+	branch = main
 [submodule "3rdparty/ffs"]
 	path = 3rdparty/ffs
 	url = ../ffs.git