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.com
Signed-off-by: Mark Brown <broonie@kernel.org>