bus: fsl-mc: dpio: add DPIO driver overview document

add document describing the dpio driver and it's role, components
and major interfaces

Signed-off-by: Stuart Yoder <stuart.yoder@nxp.com>
Signed-off-by: Roy Pledge <roy.pledge@nxp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>