xlnx-ep108: Rename to ZCU102

The EP108 is a early access development board. Now that silicon is in
production people have access to the ZCU102. Let's rename the internal
QEMU files and variables to use the ZCU102.

There is no functional change here as the EP108 is still a valid board
option.

Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
