Docs: Update out-of-date description for RoT Services

FF-M updates the description about whether an RoT Service is a
Secure Partition RoT service. Update the implementation document
to address the change.

Signed-off-by: Ken Liu <Ken.Liu@arm.com>
Change-Id: Ia150f8ffdfbd77d90a8b0b13193dc7df6ba434be
1 file changed