net: mvpp2: Add parsing support for different IPv4 IHL values

Add parser entries for different IPv4 IHL values.
Each entry will set the L4 header offset according to the IPv4 IHL field.
L3 header offset will set during the parsing of the IPv4 protocol.

Because of missed parser support for IP header length > 20, RX IPv4 checksum HW offload fails
and skb->ip_summed set to CHECKSUM_NONE(checksum done by Network stack).
This patch adds RX IPv4 checksum HW offload capability for frames with IP header length > 20.

v1 --> v2
- Improve commit message.

Suggested-by: Dana Vardi <danat@marvell.com>
Signed-off-by: Stefan Chulski <stefanc@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>