arm64: dts: rockchip: Fix RK3328 UART DMAs

Using a serial console on RK3328 provokes an error from
of_dma_request_slave_channel() since the UART nodes have a "dmas"
property but are missing the mandatory "dma-names" to go with it.

Replace the bogus "#dma-cells" - these UARTs are DMA channel consumers,
not providers - with the appropriate names instead. DMA still doesn't
actually work, since the PL330 driver doesn't quite implement everything
the 8250 driver demands, but at least it makes the DT correct.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>