drivers: qcom: rpmh-rsc: Check cmd_db_ready() to help children

Children of RPMh will need access to cmd_db.  Rather than having each
child have code to check if cmd_db is ready let's add the check to
RPMh.

With this we'll be able to remove this boilerplate code from
clk-rpmh.c and qcom-rpmh-regulator.c.  Neither of these files has
landed upstream yet but patches are pretty far along.

===
This code is based upon v11 of Lina and Raju's RPMh series.

Suggested-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Acked-by: Lina Iyer <ilina@codeaurora.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>