Xilinx: ARM: making u-boot match FSBL for zc770

Xilinx: ARM: making u-boot match FSBL for zc770

This should allow u-boot to be built for the zc770 and
work with the default FSBL which setups up the DDR, PLLs,
and MIO for 800 MHz CPU, 533 MHz DDR.

UART and Ethernet are working at this point.