dt-bindings: firmware: scm: add sdm670 compatible

The Snapdragon 670 uses SCM as for PSCI power management. Document the
appropriate compatible string for it.

Signed-off-by: Richard Acayan <mailingradian@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Guru Das Srinagesh <quic_gurus@quicinc.com>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20221111001818.124901-4-mailingradian@gmail.com