ARM: dts: Enable UART wake-up events for beagleboard

For device tree based booting, we need to use wake-up
interrupts like we already do for some omaps. This fixes
a PM regression on beagleboard compared to legacy booting.

Tested-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>