docs: Add kernel-doc comments to VME driver API

Add kernel-doc comments to the VME driver API and structures. This
documentation will be integrated into the RST documentation in a later
patch.

Signed-off-by: Martyn Welch <martyn.welch@collabora.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>