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>

Documentation/driver-api/index.rst

@@ -92,7 +92,6 @@ available subsections can be seen below.
    pwm
    rfkill
    serial/index
-   sgi-ioc4
    sm501
    smsc_ece1099
    switchtec