dt-bindings: hwmon: ltc2991: add bindings

Add dt-bindings for ltc2991 octal i2c voltage, current and temperature
monitor.

Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20231026103413.27800-1-antoniu.miclaus@analog.com


Signed-off-by: Guenter Roeck <linux@roeck-us.net>