Add virtual i2c driver to send SRTM i2c messages to M4.
Each virtual I2C bus has a specal bus id, which is abstracted by M4.
Each SRTM message include a bus id for the bus which the device is on.

Virtual i2c rpmsg bus will bind rpbus nodes with compatible string
"fsl,i2c-rpbus". And "rpmsg-i2c-channel" will probe only one rpmsg
channel for all rpbuses.

This virtual i2c driver depends on CONFIG_I2C and CONFIG_RPMSG.

Signed-off-by: Clark Wang <xiaoning.wang@nxp.com>
(cherry picked from commit 9feeac93)
(cherry picked from commit 379ab839)