net: Support partial checksum offloading

The Xilinx AXI Ethernet subsystem is capable of RX/Tx checksum
offloading. While it supports computing IP and UDP/TCP checksums, it
does not support computing ICMP checksums and only computes IP checksums
for ICMP messages. Thus, this patch adds an additional configuration for
ethernet drivers that indicates for which protocols checksum offloading
is (to be) supported. This flag is then considered by the IP subsystem
in determining when flags need to be computed in software.

Signed-off-by: Eric Ackermann <eric.ackermann@cispa.de>