dt-bindings: arm: axiado: add AX3000 EVK compatible strings

Add device tree binding schema for Axiado platforms, specifically the
AX3000 SoC and its associated evaluation board. This binding will be
used for the board-level DTS files that support the AX3000 platforms.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Harshit Shah <hshah@axiado.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>