tf-cov-make: use 'main' branch for TF-M tests and extras repo

Switch to the 'main' branch for TF-M tests and the extras
repository, in response to the recent changes [1] in TF-A.

[1]: https://review.trustedfirmware.org/c/TF-A/trusted-firmware-a/+/26263

Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
Change-Id: I0c75a712c570e70cc19d12f473d534777d7183d4
1 file changed