openrisc: Remove kernel-doc marker from ioremap comment

Replace the kernel-doc marker (/**) with a regular comment to fix the
warning:

  arch/openrisc/mm/ioremap.c:108: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst

Signed-off-by: Stafford Horne <shorne@gmail.com>