power: supply: ltc2941-battery-gauge: Add vendor to compatibles in binding

The DT binding document for LTC2941 and LTC2943 battery gauges did not use
a vendor prefix in the listed compatible strings. The driver says that the
manufacturer is Linear Technology which is "lltc" in vendor-prefixes.txt.

There isn't an upstream Device Tree source file that has nodes defined for
these devices, so there's no need to keep the old compatible strings.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sre@kernel.org>