MLK-21023 i2c: rpmsg-imx: add mutex lock when transfer i2c messages

I2c_lock_bus function in i2c-core-base will not stop the transfer to
different devices on different buses at the same time.

Since the multiple rpmsg i2c buses share one rpmsg channel, so it has to
add mutex to protect rpmsg resource accessing.

Signed-off-by: Clark Wang <xiaoning.wang@nxp.com>
(cherry picked from commit d592afe9)
(cherry picked from commit 44622ff8)