rk3399: Add ROC-RK3399-PC Mezzanine board

Add Firefly ROC-RK3399-PC Mezzanine board which is an
extension board on top of roc-rk3399-pc.

Will drop the separate defconfig file, once we support
the board detection at runtime.

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