omap2plus: ensure that one of omap2/3/4 is selected

Random configurations can fail if none of the omap families
in mach-omap2 is selected. This patch automatically selects
omap2 if the user has not made any other choice.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>