Documentation: DT: Add entry for DPAA2 console

This patch adds a devicetree binding documentation for
FSL's DPAA2 console.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Li Yang <leoyang.li@nxp.com>