mips: octeon: nic23: Enable ethernet support

mips: octeon: nic23: Enable ethernet support

This patch enables the Kconfig symbols needed for full ethernet support
on the NIC23. Additionally board specific setup is done, mostly GPIOs
related to SFP / GPIO configuration. With this, ethernet can be used on
this board. Here an example of a tftp load:

=> tftp ffffffff81000000 big
Using ethernet-mac-nexus@11800e2000000 device
TFTP from server 192.168.1.5; our IP address is 192.168.1.247
Filename 'big'.
Load address: 0xffffffff81000000
Loading: ##################################################  10 MiB
         9.7 MiB/s
done
Bytes transferred = 10485760 (a00000 hex)

Signed-off-by: Stefan Roese <sr@denx.de>