net: mvpp2: introduce PPv2.2 HW descriptors and adapt accessors

This commit adds the definition of the PPv2.2 HW descriptors, adjusts
the mvpp2_tx_desc and mvpp2_rx_desc structures accordingly, and adapts
the accessors to work on both PPv2.1 and PPv2.2.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>