yocto/kas: Pin meta-cassini to last working SHA

meta-cassini commit e3e886ba enables a Trusted Services-specific Parsec
configuration file based on the "cassini" distro only, without regard to
the recipe-level PACKAGECONFIG, which may specify a different backend.

Until the Kronos Trusted Services backend is ready, pin to the last
working SHA.

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