docs: Get the struct cmbdata kernel doc from the right file

Back in 2012, commit 9807f759 (UAPI: (Scripted) Disintegrate
arch/s390/include/asm) moved struct cmbdata (and its kerneldoc comments) to
another file, but did not update the docs to match.  The result is this
warning:

    ./arch/s390/include/asm/cmb.h:1: warning: no structured comments found

...and no documentation for that structure.  Update the docs to get the
information from the right place.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>