regmap: Cache single values read from the chip

If we don't have a cached value for a register and we can cache it then
when we do a read a value we should add it to the cache to save rereading
it later on. Do this for single register reads, for block reads the code
would be a little more complex and this covers most practical usage.
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

drivers/base/regmap/regmap.c

@@ -698,6 +698,9 @@ static int _regmap_read(struct regmap *map, unsigned int reg,
 		trace_regmap_reg_read(map->dev, reg, *val);
 	}
 
+	if (ret == 0 && !map->cache_bypass)
+		regcache_write(map, reg, *val);
+
 	return ret;
 }