arm64: dts: allwinner: Enable button wakeup on Orange Pi PC2

The Orange Pi PC2 features a GPIO button. As the button is connected to
Port L (pin PL3), it can be used as a wakeup source. Enable this.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>