ARM: dts: OMAP3: beagle/overo: mux console UART, enable wakeup

Ensure the console uart (UART3) on these boards is mux'd correctly, and
IO ring wakeup is enabled.

This is needed for serial console wakeups when using DT boot.

Thanks to Florian Vaussard for suggestion to use preprocessor
features.

Cc: Florian Vaussard <florian.vaussard@epfl.ch>
Signed-off-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Benoit Cousson <benoit.cousson@linaro.org>