regmap: add regmap_field_update_bits()

Current regmap_field is supporting read/write functions.
This patch adds new update_bits function for it.

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

diff --cc drivers/base/regmap/regmap.c
Simple merge

diff --cc include/linux/regmap.h
Simple merge