zynq-zc702-adv7511.dts: Fix HDMI audio DMA channel

DTS was still using old syntax, this fixes the HDMI audio failing
with EINVAL on any attempt to playback audio.
zynq-zed.dtsi: Reduce SD clock frequency from 50 to 33 MHz.

Zedboard often failed to boot as a result of the SD frequency being
too high. Add a label to the sdcard interface and override its
frequency in zynq-zed.dtsi.