soc: qcom: rpmh-rsc: Document the register layout better

Perhaps it's just me, it took a really long time to understand what
the register layout of rpmh-rsc was just from the #defines.  Let's add
a bunch of comments describing which blocks are part of other blocks.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Maulik Shah <mkshah@codeaurora.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Link: https://lore.kernel.org/r/20200413100321.v4.2.Iaddc29b72772e6ea381238a0ee85b82d3903e5f2@changeid


Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>