ARM: omap2plus_defconfig: Enable support for RTC M41T80

The phyCORE-AM335x SoM has a RV4162 RTC populated which is supported
by the M41T80 driver. Enabled it so make the RTC support on the SoM
available.

Signed-off-by: Teresa Remmet <t.remmet@phytec.de>
Signed-off-by: Tony Lindgren <tony@atomide.com>