OPP: Fix required_opp_tables for multiple genpds using same table

The required_opp_tables parsing is not perfect, as the OPP core does the
parsing solely based on the DT node pointers.

The core sets the required_opp_tables entry to the first OPP table in
the "opp_tables" list, that matches with the node pointer.

If the target DT OPP table is used by multiple devices and they all
create separate instances of 'struct opp_table' from it, then it is
possible that the required_opp_tables entry may be set to the incorrect
sibling device.

Unfortunately, there is no clear way to initialize the right values
during the initial parsing and we need to do this at a later point of
time.

Cross check the OPP table again while the genpds are attached and fix
them if required.

Also add a new API for the genpd core to fetch the device pointer for
the genpd.

Cc: Thorsten Leemhuis <regressions@leemhuis.info>
Reported-by: Vladimir Lypak <vladimir.lypak@gmail.com>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=218682


Co-developed-by: Vladimir Lypak <vladimir.lypak@gmail.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>