adc: Move the zynq-xadc file to the adc directories

adc: Move the zynq-xadc file to the adc directories

It's an ADC, put it where it belongs.

Cc: Edgar E. Iglesias <edgar.iglesias@gmail.com>
Cc: Alistair Francis <alistair@alistair23.me>
Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: qemu-arm@nongnu.org
Signed-off-by: Corey Minyard <cminyard@mvista.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>