regulator: qcom-rpmh: Support RPMH address reads and use it for rpmh-regulators

Kamal Wadhwa <kamal.wadhwa@oss.qualcomm.com> says:

This patch series adds a new `rpmh_read()` API to allow reading RPMH
addresses. Using this API enhances the RPMH regulator driver by adding
readback of the voltage/bypass/mode settings as they have been applied by
APPS during the bootloader stage, so regulator framework can get them
via `get_mode`, `get_bypass` & `get_voltage_selector` callbacks during
regulator registration.

This is needed because currently regulator framework does a unnecessary
write with `min-microvolt` DT setting for all the RPMH regulators during
regulator registration, because the first time after boot the value is
seen as -ENOTRECOVERABLE, as there is no option to read these regulator
settings.

With this change this unnecessary write can be avoided and regulator
framework gets a sense of the initial state set during the bootloader
stage for all regulator settings.

NOTE - During discussion on the v2 series - PATCH 3/4, reviewer had
inquired about possible need for the use of the sync_state() to handle the
"multiple" client case - for maintaining the regulator settings till all
the clients are probed.

This case was not covered in my previous series and had originally planned
to do that series separately. But after the discussion decided to merge
the 2 series as it seemed this would be a better approach. But after
working on sync_state change. I realized a basic issue with using
sync_state() for regulators - that its per-driver and not per-regulator
resource. But we needed a sync_state callback for each regulator separately.

I had been experimenting with few ideas but seems its going to need more
time for me to close on the equivalent solution that has per-regulator
sync_state or something to that effect. So I thought to close on this
series and attend to that separately.

Link: https://patch.msgid.link/20260801-b4-read-rpmh-v5-v6-0-9fcb54928523@oss.qualcomm.com

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