ARM: dts: qcom: msm8974-hammerhead: add device tree bindings for mpu6515

This patch adds device tree bindings for the mpu6515 to the LG Nexus 5
(hammerhead) phone. Confirmed that the gyroscope / accelerometer
(mpu6515), magnetometer (ak8963), and temperature / pressure (bmp280)
sensors are available on the phone.

Interrupts are not working properly on the ak8963 magnetometer so they
are currently not configured.

The bmp280 retuns temperature/pressure measurement skipped errors but
will reliably work if I run:

    echo 1 > in_pressure_oversampling_ratio
    echo 1 > in_temp_oversampling_ratio

Signed-off-by: Brian Masney <masneyb@onstation.org>
Signed-off-by: Jonathan Marek <jonathan@marek.ca>
Signed-off-by: Andy Gross <andy.gross@linaro.org>