regmap: add regmap_write_bits()

regmap_write_bits() is similar to regmap_update_bits(),
but regmap_write_bits() write data to register even though
it is same value.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>