Add helper functions to use the i2c auxiliary bus with the MPU i2c
master block.

Support only register based chip, reading and 1 byte writing. These
will be useful for initializing magnetometers inside MPU9x50 chips.
Signed-off-by: Jean-Baptiste Maneyrol <jmaneyrol@invensense.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>