Add top-level.yml to U-Boot CI

[ozan956]

In the workflows, the Check defconfigs job in checks.yml points to the top-level.yml which is not exist.

See: https://github.com/analogdevicesinc/u-boot/actions/runs/23156057016/job/67270991801#step:9:15

[pamolloy]

See 7f41c45

[ozan956]

We already have main.yml that is actually can be used to define the defconfigs.

So I defined all in this PR:
#76

Also, I have replaced make defconfig based config file check with qconfig.py based check in here:
#70

These two PRs should be merged together in order to have functional CI flow.

[ozan956]

The top-level.yml file is originally used on the kernel side to define build jobs.

In U-Boot, however, we are not building every defconfig available for the ADSP platform. Therefore, we have two possible approaches: either we merge these two PRs as they are, or we introduce a top-level.yml and define build jobs for all boards. In that case, I can update the check_assert_defconfigs function accordingly.