Documentation: devicetree: Add property for controlling power saving mode for the us5182 als sensor

Add a property to allow changing the default power-saving mode.
By default, at read raw the chip will activate and provide
one measurent, then it will shut itself down. However, the
chip can also work in "continuous" mode which may be more reliable
but is also more power consuming.

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