Documentation: Add io_ordering.rst to driver-api manual

Add io_ordering.rst under Documentation/driver-api and reference it from
the Sphinx TOC Tree present in Documentation/driver-api/index.rst
Signed-off-by: Pragat Pandya <pragat.pandya@gmail.com>
Link: https://lore.kernel.org/r/20200303050301.5412-3-pragat.pandya@gmail.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/driver-api/index.rst

@@ -80,6 +80,7 @@ available subsections can be seen below.
    isa
    isapnp
    io-mapping
+   io_ordering
    generic-counter
    lightnvm-pblk
    memory-devices/index