u-boot: Fix URI and rename to xilinx-zynq

The "u-boot-xarm" repository is gone, use "u-boot-xlnx" instead. (Thanks Corey for the suggestion)
Also name the version "zynq" instead of "zc702" because it's for both zedboard and zc702.
machine conf: Fix rtc feature, only set FSTYPES once

No need to duplicate FSTYPES types settings.
Only the zc702 has an rtc, the zedboard does not. So only put "rtc"
in the MACHINE_FEATURES for the zc702, and tell OE that we know that
the rtc feature exists even though we don't use it on the zedboard.
zedboard devicetree: Set up LD9 (not working)

LD9 is connected to MIO7 which should mean that the LED can be under
generic control. However, gpio 7 appears to be claimed by something and
it doesn't work. Apparently the SD driver is using this line, but it
is not working as MMC activity indication either.