dt-bindings: soc: qcom: spm: Document SDM660 and MSM8998 compatibles

The driver was updated to add SAW2 v4.1 support for new SoCs: document
the new compatibles.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@somainline.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20210729155609.608159-6-angelogioacchino.delregno@somainline.org