binding for LTC2941, LTC2942, LTC2943 and LTC2944 battery gauges

All chips measure battery capacity.
The LTC2942 is pin compatible with the LTC2941, it adds voltage and
temperature monitoring, and is runtime detected. LTC2943 and LTC2944
is software compatible, uses a slightly different conversion formula
for the charge counter and adds voltage, current and temperature monitoring.

Required properties:
- compatible: Should contain "lltc,ltc2941", "lltc,ltc2942", "lltc,ltc2943"
    or "lltc,ltc2944" which also indicates the type of I2C chip attached.
- reg: The 7-bit I2C address.
- lltc,resistor-sense: The sense resistor value in milli-ohms. Can be a 32-bit
    negative value when the battery has been connected to the wrong end of the
    resistor.
- lltc,prescaler-exponent: The prescaler exponent as explained in the datasheet.
    This determines the range and accuracy of the gauge. The value is programmed
    into the chip only if it differs from the current setting. The setting is
    lost when the battery is disconnected.

Example from the Topic Miami Florida board:

	fuelgauge: ltc2943@64 {
		compatible = "lltc,ltc2943";
		reg = <0x64>;
		lltc,resistor-sense = <15>;
		lltc,prescaler-exponent = <5>; /* 2^(2*5) = 1024 */
	};