staging: fsl-dpaa2/eth: convert documentation to .rst format

Convert the DPAA2 Ethernet driver documentation to .rst format
and rename the file accordingly.
Also add a SPDX tag to the new rst file.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>