soc: qcom: smd-rpm: Add sdm660 compatible

Add the compatible for the RPM in SDM660, so that rpm resources can be
made available.

Signed-off-by: Craig Tatlor <ctatlor97@gmail.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>