Upgrade u-boot, load FPGA image from u-boot for zynq-miami

Upgrade u-boot to master-next to incorporate extra fixes and extensions.

The zymq-miami boards need early access to logic. Instead of using SPL to
load the FPGA, do this from the "big" u-boot. This should be a bit faster,
and allows more flexibility (such as loading the FPGA from flash).