ARM: dts: colibri-imx6: improve wake-up with gpio key

The pin GPIO2_IO22 externally pulls down, it is required to sequentially
connect this pin (signal WAKE_MICO#) to +3v3 and then disconnect it to
trigger a wakeup interrupt.
Adding the flag GPIO_PULL_DOWN allows the system to be woken up just
connecting the pin GPIO2_IO22 to +3v3.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>