i2c: add slave testunit driver

Here is an I2C slave backend driver which allows to test some uncommon
functionalities of the I2C and SMBus world. Usually, you need specific
devices to test e.g. SMBus Host Notify and such. With this driver you
just need the slave interface of another I2C controller.

This initial version has testcases for multi-master and SMBus Host
Notify. Already planned but not yet implemented are SMBus Alert and
messages with I2C_M_RECV_LEN.

Please read the documentation for further details.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>