regmap: warn users about uninitialized flat cache

regmap: warn users about uninitialized flat cache

The standard flat cache did not contain any validity info, so the cache
was always considered to be entirely valid. Multiple mechanisms exist to
initialize the cache on regmap init (defaults, raw defaults, HW init),
but not all drivers are using one of these. As a result, their
implementation might currently depend on the zero-initialized cache or
contain other workarounds.

When reading an uninitialized value from the flat cache, warn the user,
but maintain the current behavior. This will allow developers to switch
to a sparse (flat) cache independently.

Signed-off-by: Sander Vanheule <sander@svanheule.net>
Link: https://patch.msgid.link/20251029081248.52607-3-sander@svanheule.net
Signed-off-by: Mark Brown <broonie@kernel.org>