config/qa-checks: Introduce spell dictionary

Update qa-checks_config.yml to include spell check.
And add 'layer' into default_check_excludes as well.

Issue-Id: SCM-4874
Signed-off-by: Robbie Cao <robbie.cao@arm.com>
Change-Id: I4dda3ce07823429d9e9e393c48205e770d7d958c