i2c: mt7621: Add MediaTek MT7621/7628/7688 I2C driver

This patch adds a driver for the I2C controller found on the MediaTek
MT7621/7628/7688 SoC's. The base version of this driver was done by
Steven Liu (according to the copyright and MODULE_AUTHOR lines). It
can be found in the OpenWRT repositories (v4.14 at the time I looked).

The base driver had many issues, which are disccussed here:

https://en.forum.labs.mediatek.com/t/openwrt-15-05-loads-non-working-i2c-kernel-module-for-mt7688/1286/3

>From this link an enhanced driver version (complete rewrite, mayor
changes: support clock stretching, repeated start, ACK handling and
unlimited message length) from Jan Breuer can be found here:

https://gist.github.com/j123b567/9b555b635c2b4069d716b24198546954

This patch now adds this enhanced I2C driver to mainline.

Changes by Stefan Roese for upstreaming:
- Add devicetree bindings
- checkpatch clean
- Use module_platform_driver()
- Minor cosmetic enhancements
- Removed IO warpped functions
- Use readl_relaxed_poll_timeout() and drop poll_down_timeout()
- Removed superfluous barrier() in mtk_i2c_reset()
- Use i2c_8bit_addr_from_msg()
- Added I2C_FUNC_PROTOCOL_MANGLING
- Removed adap->class = I2C_CLASS_HWMON | I2C_CLASS_SPD;

Signed-off-by: Stefan Roese <sr@denx.de>
Tested-by: René van Dorst <opensource@vdorst.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>