powerpc/configs: Add (back) MLX5 ethernet support to skiroot_defconfig

It turns out that some defconfig changes and kernel config option
changes meant we accidentally dropped Ethernet support for Mellanox
CLX5 cards.

Fixes: cbc39809 ("powerpc/configs: Update skiroot defconfig")
Reported-by: Carol L Soto <clsoto@us.ibm.com>
Suggested-by: Carol L Soto <clsoto@us.ibm.com>
Signed-off-by: Stewart Smith <stewart@linux.ibm.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>