dt-bindings: arm: qcom: add sdm670 and pixel 3a compatible

The Snapdragon 670 and Pixel 3a initial device trees are about to be
added. Add the compatible strings for the board.

Signed-off-by: Richard Acayan <mailingradian@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20221111001818.124901-3-mailingradian@gmail.com