ARM: dts: imx6: make edt-ft5x06 a wakeup source for imx6 boards

The touchscreen driver no longer configures the device as wakeup source by
default. A "wakeup-source" property is needed.

Signed-off-by: Daniel Mack <daniel@zonque.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>