sensor: current_amp: optional high-range gain fallback

Add the option to specify an alternate ADC gain value to use if the
initial measurement saturates the range. This enables higher data
resolutions when the values are small compared to the maximum signal
values, while still supporting the maximum.

As a concrete example, measuring charge currents from a small solar
panel (0 - 50mA), while also supporting high USB charge currents
(up to 1A).

Signed-off-by: Jordan Yates <jordan@embeint.com>