dt-bindings: arm: qcom: Document msm8956 and msm8976 SoC and devices

Note that msm8976 is omitted as a compatible, since there are currently
no boards/devices using it.

Signed-off-by: Marijn Suijten <marijn.suijten@somainline.org>
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20221111120156.48040-9-angelogioacchino.delregno@collabora.com