dt-bindings: arm: rockchip: Update ROCKPi 4 binding

dt-bindings: arm: rockchip: Update ROCKPi 4 binding

ROCKPi 4 has 3 variants of hardware platforms called
ROCKPi 4A, 4B, and 4C.

- ROCKPi 4A has no Wif/BT.
- ROCKPi 4B has AP6256 Wifi/BT, PoE.
- ROCKPi 4C has AP6256 Wifi/BT, PoE, miniDP, USB Host enabled
  GPIO pin change compared to 4B, 4C

So, update the existing ROCKPi 4 binding to support
ROCKPi 4A/B/C hardware platforms.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200807094826.12019-1-jagan@amarulasolutions.com
Signed-off-by: Heiko Stuebner <heiko@sntech.de>