ARM: dts: rockchip: add root compatible properties

Add the missing 'compatible' property to device tree root node of

 - rk3066a-bqcurie2.dts
 - rk3188-radxarock.dts

and document the new values.

Signed-off-by: Beniamino Galvani <b.galvani@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>