regmap: Add bus reg_update_bits() support

Add reg_update_bits() support in case some platforms use a special method
to update bits of registers.
Signed-off-by: Baolin Wang <baolin.wang7@gmail.com>
Link: https://lore.kernel.org/r/df32fd0529957d1e7e26ba1465723f16cfbe92c8.1586757922.git.baolin.wang7@gmail.comSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/base/regmap/regmap.c

@@ -827,6 +827,7 @@ struct regmap *__regmap_init(struct device *dev,
 	} else if (!bus->read || !bus->write) {
 		map->reg_read = _regmap_bus_reg_read;
 		map->reg_write = _regmap_bus_reg_write;
+		map->reg_update_bits = bus->reg_update_bits;
 
 		map->defer_caching = false;
 		goto skip_format_initialization;