iio: imu: inv_mpu6050: Add i2c mux for by pass

This chip allows some limited number of sensors connected to it as
slaves, which can be directly accessed by register interface of this
driver.But the current upstream driver doesn't support such mode.
To attach such slaves to main processor i2c bus, chip has to be set
up in bypass mode. This change adds i2c mux, which will enable/disable
this mode for transaction to/from such slave devices.
This was discussed for a while in mailing list, this was the outcome:
Reference:
http://www.spinics.net/lists/linux-iio/msg12126.html
http://comments.gmane.org/gmane.linux.kernel.iio/11470



Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Reviewed-by: Wolfram Sang <wsa@the-dreams.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>