ARM: dts: OMAP3: beagle: enable user button via gpio_keys, enable wakeup

Using the gpio-keys bindings, configure the user button on Beagle
boards.  Since the user button is enabled as a wakeup source, also
ensure the GPIO pin is mux'd correctly and has IO ring wakeups enabled,
so it can also wakeup from off mode.

Special thanks to Florian Vaussard for suggesting the preprocessor
feature.

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