linux-phytec-ti

docs: rapidio: add it to the driver API

This is actually a subsystem description, with contains both
kAPI and uAPI.

While it should ideally be slplit, let's place it at driver-api,
as most things are related to kAPI and driver-specific info.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>