arm64: dts: rockchip: Mark rock-pi-4 as rock-pi-4a dts

arm64: dts: rockchip: Mark rock-pi-4 as rock-pi-4a dts

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 move common nodes, properties into dtsi file and include
on respective variant dts files.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Link: https://lore.kernel.org/r/20200807094826.12019-2-jagan@amarulasolutions.com
Signed-off-by: Heiko Stuebner <heiko@sntech.de>