ARM: dts: msm: Add the register offset for modem PDC on SDM670 and SDM845

The offset in the PDC register for modem can change between different
chipsets based on number of subsystems and may also change between
different versions of chipsets. Add the offset information in the DT.

Change-Id: Ib7b4220b90f49ef8554f9cde0d27ab79e2c84606
Signed-off-by: Kyle Yan <kyan@codeaurora.org>