board: roc-pc-rk3399: Add support for onboard LED's and push button to indicate power mode

Added support for onboard LED's and push button. When powered board will be
in low power mode(yellow LED), on button press, board enters full power mode
(red LED) and boots u-boot.

Signed-off-by: Suniel Mahesh <sunil@amarulasolutions.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>