iio: light: us5182d: Add property for choosing default power mode

This chip supports two power modes.
1. "one-shot" mode - the chip activates and executes one complete
conversion loop and then shuts itself down. This is the default mode
chosen for raw reads.
2. "continuous" mode - the chip takes continuous measurements.

Continuous mode is more expensive power-wise but may be more reliable.
Add a property so that if preferred, the default power mode for raw
reads can be set to continuous.
Separate one-shot enabling in a separate function that will be used
depending on the chosen power mode. Also create a function for
powering the chip on and off.

Signed-off-by: Adriana Reus <adriana.reus@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>