power: supply: max17040: Config alert SOC low level threshold from FDT

For configuration of fuel gauge alert for a low level state of charge
interrupt we add a function to config level threshold and a device tree
binding property to set it in flatned device tree node.

Now we can use "maxim,alert-low-soc-level" property with the values from
1% up to 32% to configure alert interrupt threshold.

Signed-off-by: Matheus Castello <matheus@castello.eng.br>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>