regmap: Add a helper function for regcache sync test

We're going to add another "does this register need syncing?" check, so
rather than repeating it in three places, we'll separate all of the
relevant logic into a helper function.

Signed-off-by: Kevin Cernekee <cernekee@chromium.org>
Signed-off-by: Mark Brown <broonie@kernel.org>
(cherry picked from commit 3969fa08)