Documentation: Add io-mapping.rst to driver-api manual

Add io-mapping.rst under Documentation/driver-api and reference it from
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/20200303050301.5412-2-pragat.pandya@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>