i2c: move OF helpers into the core

i2c: move OF helpers into the core

I2C of helpers used to live in of_i2c.c but experience (from SPI) shows
that it is much cleaner to have this in the core. This also removes a
circular dependency between the helpers and the core, and so we can
finally register child nodes in the core instead of doing this manually
in each driver. So, fix the drivers and documentation, too.

[Modified to keep a stub device registration API exposed to drivers in
order to avoid disruption]

Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
(cherry picked from commit 687b81d083c082bc1e853032e3a2a54f8c251d27)
Signed-off-by: Mark Brown <broonie@kernel.org>
Conflicts:
	drivers/i2c/busses/i2c-designware-platdrv.c
	drivers/i2c/busses/i2c-imx.c
	drivers/i2c/busses/i2c-stu300.c
	drivers/i2c/busses/i2c-wmt.c
	drivers/of/Kconfig
	drivers/of/Makefile
	drivers/staging/imx-drm/imx-tve.c
	sound/soc/fsl/imx-wm8962.c