iio: imu: inv_mpu6050: Use regmap instead of i2c specific functions

Use regmap instead of i2c specific functions.
This is in preparation of splitting this driver into core and
i2c specific functionality.

Signed-off-by: Adriana Reus <adriana.reus@intel.com>
Acked-by: Crt Mori <cmo@melexis.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>