dt-bindings: arm: add xiaomi,sagit board based on msm8998 chip

Add xiaomi,sagit board (Xiaomi Mi 6) binding.

Signed-off-by: Dzmitry Sankouski <dsankouski@gmail.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20221112203300.536010-2-dsankouski@gmail.com