dt-bindings: soc: xilinx: Move xilinx.yaml from arm to soc

All Xilinx boards can hosts also soft core CPUs like MicroBlaze or
MicroBlaze V (RISC-V ISA) that's why move boards description from arm
folder to soc folder.
Similar change was done for Renesas by commit c27ce08b

 ("dt-bindings:
soc: renesas: Move renesas.yaml from arm to soc").

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Michal Simek <michal.simek@amd.com>