arm64: dts: sdm845: Include rpmpd DT header

In order to fix dependencies with rpmpd DT entries, the header was
dropped and hardcoded values were added for opp-level, during the
previous merge window.
Add the header back in now and remove the hardcodings, effectively
reverting commit '08585d21: arm64: dts:
sdm845: Fixup dependency on RPMPD includes'

Signed-off-by: Rajendra Nayak <rnayak@codeaurora.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>