Add devicetree binding documentation for the LTC2941/LTC2943 driver

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 "ltc2941" or "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 = "ltc2943";

		reg = <0x64>;

		lltc,resistor-sense = <15>;

		lltc,prescaler-exponent = <5>; /* 2^(2*5) = 1024 */

	};