binding for LTC2941 and LTC2943 battery gauges

Both the LTC2941 and LTC2943 measure battery capacity.
The LTC2943 is compatible with the LTC2941, it adds voltage and
temperature monitoring, and uses a slightly different conversion
formula for the charge counter.

Required properties:
- compatible: Should contain "lltc,ltc2941" or "lltc,ltc2943" 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 */
	};
