ARM: dts: rockchip: Add UART DMA support for rv1108

Pass the 'dmas' property to the UART ports so that DMA can
be supported.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Tested-by: Fabio Berton <fabio.berton@ossystems.com.br>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>