docs: networking: dpaa2: add documentation for the switch driver

Add a documentation entry for the DPAA2 switch listing its
requirements, features and some examples to go along them.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>