Build: Fix deprecated yaml.load calls

PyYAML's yaml.load() function provides arbitrary code execution before
PyYAML v4.1 and is therefore deprecated - as of v5.1 - with a single
file argument as described in

https://github.com/yaml/pyyaml/wiki/PyYAML-yaml.load(input)-Deprecation



Use yaml.SafeLoader in TF-M generator scripts.

Change-Id: I9eee8805053dd653fca946c4376be3eebd4bfd53
Signed-off-by: Mate Toth-Pal <mate.toth-pal@arm.com>