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 <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jonathan Corbet <[email protected]>

Documentation/driver-api/index.rst

@@ -79,6 +79,7 @@ available subsections can be seen below.
    ipmb
    isa
    isapnp
+   io-mapping
    generic-counter
    lightnvm-pblk
    memory-devices/index