arm64: dts: rockchip: Add ROCKPi 4A/4B support

arm64: dts: rockchip: Add ROCKPi 4A/4B support

Rock PI 4 has 3 variants of hardware platforms called
RockPI 4A, 4B, and 4C.

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

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

Use 4B dts into default rock-pi-4 defconfig until we find any
solution for dynamic detection of these variants.

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