docs: driver-api: Remove reference to sgi-ioc4

Commit f7bc6e42 ("drivers: remove the SGI SN2 IOC4 base support")
removed support for SGI SN2 IOC4 and the relevant documentation files.
Remove a leftover reference in the toctree of the driver-api
documentation to fix this sphinx error:

    Documentation/driver-api/index.rst:14: WARNING: toctree contains
        reference to nonexisting document 'driver-api/sgi-ioc4'

Fixes: f7bc6e42 ("drivers: remove the SGI SN2 IOC4 base support")
Signed-off-by: Harald Seiler <hws@denx.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>