ARM: zynq: Switch to single zynq configurations

There are a lot of zynq configurations which can be merged together and use
only one for all. The similar change has been done for ZynqMP by commit
be1b6c32 ("arm64: zynqmp: Use zynqmp_virt platform")

Build SPL with u-boot.img for zc706 like this.
export DEVICE_TREE=zynq-zc706 && make xilinx_zynq_virt_defconfig && make -j8
u-boot.img is generic for all boards.

Tested on Zybo, zc702, zc706, zc770-xm011-x16, cc108 and microzed.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>