ARM: dts: rockchip: Add LEDs via gpio-leds for Radxa Rock2 Square

Describe the two user-controllable LEDs on Rock2 Square boards.
All information have been retrieved from the schematics and the vendor
devicetree. The default-triggers mimic the behaviour of the vendor-kernel to
keep functionalities in sync.

Signed-off-by: Romain Perier <romain.perier@gmail.com>
Reviewed-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>