regmap: add SCCB support

This adds Serial Camera Control Bus (SCCB) support for regmap API that
is intended to be used by some of Omnivision sensor drivers.

The ov772x and ov9650 drivers are going to use this SCCB regmap API.

The ov772x driver was previously only worked with the i2c controller
drivers that support I2C_FUNC_PROTOCOL_MANGLING, because the ov772x
device doesn't support repeated starts.  After commit 0b964d18
("media: ov772x: allow i2c controllers without
I2C_FUNC_PROTOCOL_MANGLING"), reading ov772x register is replaced with
issuing two separated i2c messages in order to avoid repeated start.
Using this SCCB regmap hides the implementation detail.

The ov9650 driver also issues two separated i2c messages to read the
registers as the device doesn't support repeated start.  So it can
make use of this SCCB regmap.

Cc: Mark Brown <broonie@kernel.org>
Cc: Peter Rosin <peda@axentia.se>
Cc: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Cc: Wolfram Sang <wsa@the-dreams.de>
Cc: Sylwester Nawrocki <s.nawrocki@samsung.com>
Cc: Jacopo Mondi <jacopo+renesas@jmondi.org>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Hans Verkuil <hans.verkuil@cisco.com>
Cc: Sakari Ailus <sakari.ailus@linux.intel.com>
Cc: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>