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>

roc-pc-rk3399 board has one user button & three user LED's. Currently
we don't have any code support for these devices. Since button and LED's are
specific to roc-pc-rk3399 board, split it into its own board file and add code
support here.

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