ARM: dts: msm: Use the MHZ_TO_MBPS macro for sdm845

Let the DT compiler do the math for us instead of writing the
precomputed values directly into DT.

Change-Id: Iec23c7e4d84cb07511148f504bd5e505356557da
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>