iio: imu: mpu6050: Add support for vdd-supply regulator

MPU6050 has two power supply pins: VDD and VLOGIC, but the
mpu6050 driver only supports enabling one of them at the moment.
In some cases, they may need to be enabled separately.

Add an additional "vdd-supply" that stays enabled for as long as
the driver is loaded. We cannot turn off the VDD regulator during
suspend as this would cause register settings (FSR, sampling rate, ...)
to be lost.

Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Reviewed-by: Jean-Baptiste Maneyrol <jmaneyrol@invensense.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>