ARM: omap2plus_defconfig: enable NET_SWITCHDEV

The TI_CPSW_SWITCHDEV definition in Kconfig was changed from "select
NET_SWITCHDEV" to "depends on NET_SWITCHDEV", and therefore it is required
to explicitelly enable NET_SWITCHDEV config option in omap2plus_defconfig.

Fixes: 3727d259 ("arm: omap2plus_defconfig: enable new cpsw switchdev driver")
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>