mlek: Switch to public GitLab repo

Ml-Embedded-Evaluation-kit GitLab repos
are now live and public. Hence switching to
using them.

Changes done in this commit:

* Update the submodule for MLEK to use the public GitLab instance.
* Update the docs to refer to the public MLEK GitLab instance.

Signed-off-by: Ahmed Ismail <Ahmed.Ismail@arm.com>