net: mvpp2: jumbo frames support

This patch adds the support for jumbo frames in the Marvell PPv2 driver.
A third buffer pool is added with 10KB buffers, which is used if the MTU
is higher than 1518B for packets larger than 1518B. Please note only the
port 0 supports hardware checksum offload due to the Tx FIFO size
limitation.

Signed-off-by: Stefan Chulski <stefanc@marvell.com>
[Antoine: cosmetic cleanup, commit message]
Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>