ARM: OMAP2+: omap2plus_defconfig: enable omap1 rtc

The BeagleBone dev kit uses the built-in RTC module, so
it would be nice to have this built by default in the
omap2plus defconfig.

Signed-off-by: Mark Jackson <mpfj@newflow.co.uk>
Signed-off-by: Tony Lindgren <tony@atomide.com>