fuel_gauge: Repl property struct w/ union

Based on review of the similar charger driver API, it's been demonstrated
from the community that embedding a per value property type when fetching
properties. Separating off the property types from the property values
themselves also allow an array of property types to declared as static
const.

Break up fuel_gauge_property struct into a fuel_gauge_prop_val union and a
fuel_gauge_prop_t property type as inputs into fuel gauge API functions.

Signed-off-by: Aaron Massey <aaronmassey@google.com>