arm64: dts: juno: add UART DMA channel to Juno

Of the 8 peripheral request interfaces on Juno's DMA-330, two of them
are used for UART0 RX and TX. Whilst it's rather hard to type enough on
the console to reach the RX DMA threshold, TX DMA works just fine for
spewing debug output into the FIFO while the CPU does something more
useful, so hook that one up.

CC: Liviu Dudau <liviu.dudau@arm.com>
CC: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
CC: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>