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>

Documentation/admin-guide/index.rst

@@ -61,6 +61,7 @@ configure specific aspects of kernel behavior to your liking.
    parport
    md
    module-signing
+   rapidio
    sysrq
    unicode
    vga-softcursor

Documentation/admin-guide/rapidio.rst

@@ -0,0 +1,107 @@
+=======================
+RapidIO Subsystem Guide
+=======================
+
+:Author: Matt Porter
+
+Introduction
+============
+
+RapidIO is a high speed switched fabric interconnect with features aimed
+at the embedded market. RapidIO provides support for memory-mapped I/O
+as well as message-based transactions over the switched fabric network.
+RapidIO has a standardized discovery mechanism not unlike the PCI bus
+standard that allows simple detection of devices in a network.
+
+This documentation is provided for developers intending to support
+RapidIO on new architectures, write new drivers, or to understand the
+subsystem internals.
+
+Known Bugs and Limitations
+==========================
+
+Bugs
+----
+
+None. ;)
+
+Limitations
+-----------
+
+1. Access/management of RapidIO memory regions is not supported
+
+2. Multiple host enumeration is not supported
+
+RapidIO driver interface
+========================
+
+Drivers are provided a set of calls in order to interface with the
+subsystem to gather info on devices, request/map memory region
+resources, and manage mailboxes/doorbells.
+
+Functions
+---------
+
+.. kernel-doc:: include/linux/rio_drv.h
+   :internal:
+
+.. kernel-doc:: drivers/rapidio/rio-driver.c
+   :export:
+
+.. kernel-doc:: drivers/rapidio/rio.c
+   :export:
+
+Internals
+=========
+
+This chapter contains the autogenerated documentation of the RapidIO
+subsystem.
+
+Structures
+----------
+
+.. kernel-doc:: include/linux/rio.h
+   :internal:
+
+Enumeration and Discovery
+-------------------------
+
+.. kernel-doc:: drivers/rapidio/rio-scan.c
+   :internal:
+
+Driver functionality
+--------------------
+
+.. kernel-doc:: drivers/rapidio/rio.c
+   :internal:
+
+.. kernel-doc:: drivers/rapidio/rio-access.c
+   :internal:
+
+Device model support
+--------------------
+
+.. kernel-doc:: drivers/rapidio/rio-driver.c
+   :internal:
+
+PPC32 support
+-------------
+
+.. kernel-doc:: arch/powerpc/sysdev/fsl_rio.c
+   :internal:
+
+Credits
+=======
+
+The following people have contributed to the RapidIO subsystem directly
+or indirectly:
+
+1. Matt Porter\ mporter@kernel.crashing.org
+
+2. Randy Vinson\ rvinson@mvista.com
+
+3. Dan Malek\ dan@embeddedalley.com
+
+The following people have contributed to this document:
+
+1. Matt Porter\ mporter@kernel.crashing.org