ARM: dts: sun8i: h3: beelink-x2: Add power button

Beelink X2 has power button. Add node for it.

Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20210323204341.28825-1-jernej.skrabec@siol.net