arm64: dts: rockchip: Add on-board LED support on rk3399-rock960

Add on-board LED support for Rock960 board based on the following
standard used by rest of the 96Boards:

green:user1  default-trigger: heartbeat
green:user2  default-trigger: mmc0/disk-activity(onboard-storage)
green:user3  default-trigger: mmc1 (SD-card)
green:user4  default-trigger: none, panic-indicator
yellow:wlan  default-trigger: phy0tx
blue:bt      default-trigger: hci0-power

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>