powerpc/pseries: Add BNX2=m to defconfig

Upcoming servers will include a Broadcom NIC, add to the defconfig to
increase testing coverage and make sure mainline builds come up with
networking.

Signed-off-by: Nishanth Aravamudan <nacc@us.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>