regmap: permit to set reg_update_bits with bulk implementation

A regmap may still require to set a custom reg_update_bits instead of
relying to the regmap_bus_read/write general function.

Permit to set it in the map if provided by the regmap config.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Link: https://lore.kernel.org/r/20220715201032.19507-1-ansuelsmth@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>