s390: enable switchdev support in defconfig

The HiperSockets Converged Interface (HSCI) introduced with commit
4e20e73e ("s390/qeth: Switchdev event handler") requires
CONFIG_SWITCHDEV=y to be usable. Similarly when using Linux controlled
SR-IOV capable PF devices with the mlx5_core driver CONFIG_SWITCHDEV=y
as well as CONFIG_MLX5_ESWITCH=y are necessary to actually get link on
the created VFs. So let's add these to the defconfig to make both types
of devices usable. Note also that these options are already enabled in
most current distribution kernels.

Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>