power: supply: ltc2941-battery-gauge: Add LTC2942 support

LTC2942 is pin compatible with LTC2941 providing additional
informations about battery voltage and temperature. It can
be runtime detected using bit A7 in the Status register.

Signed-off-by: Ladislav Michl <ladis@linux-mips.org>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Dragos Bogdan <dragos.bogdan@analog.com>
Tested-by: Dragos Bogdan <dragos.bogdan@analog.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>