iio: imu: mpu6050: add support for 6515 variant

This patch adds support for the MPU 6515 variant. Confirmed that the
driver functions correctly on a LG Nexus 5 (hammerhead) phone.

Signed-off-by: Brian Masney <masneyb@onstation.org>
Signed-off-by: Jonathan Marek <jonathan@marek.ca>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>