regmap: Implement generic syncing functionality

In the absence of a sync callback, do it manually.  This of course
can't take advantange of the specific optimizations of each
cache type but it will do well enough in most cases.

Signed-off-by: Dimitris Papastamos <dp@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>