docs: move core-api/ioctl.rst to driver-api/

The ioctl() documentation belongs with the rest of the driver-oriented
info, so move it there.

Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/core-api/index.rst

···
    :maxdepth: 1
 
    librs
-   ioctl
 
 .. only:: subproject and html
 

Documentation/driver-api/index.rst

···
    driver-model/index
    basics
    infrastructure
+   ioctl
    early-userspace/index
    pm/index
    clk