docs: fix pointers to io-mapping.rst and io_ordering.rst files

docs: fix pointers to io-mapping.rst and io_ordering.rst files

Those files got moved, but cross-references still point to the
wrong places.

Fixes: 387ecb6a34dc ("Documentation: Add io-mapping.rst to driver-api manual")
Fixes: aad1fa68a141 ("Documentation: Add io_ordering.rst to driver-api manual")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Reviewed-by: Alex Shi <alex.shi@linux.alibaba.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/c0205119db4fef536272cb0a183b6c14c2c8bf4c.1583927470.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>