cassini-distro: Use PARSEC_CONFIG to configure Parsec

The parsec-service recipe provides the ability to override the config
file installed in the package by setting the PARSEC_CONFIG variable to
an alternate file.

Provide a dedicated config file for the Trusted Services provider
instead of patching the in-tree config file.

Changelog: other
Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>