hwmon: (pwm-fan) Support multiple fan tachometers

The pwm-fan driver is extended to support multiple fan tachometer
signals connected to GPIO inputs. This is intended to support the case
where a single PWM output signal is routed to multiple fans, each of
which have a tachometer output connected back to a GPIO pin.

The number of fan tachometer inputs is determined by the number of
interrupt sources configured for the pwm-fan device. The number of
pulses-per-revolution entries should match the number of interrupt
sources so that each input has a value assigned.

The fan tachometer measurements are exposed as sysfs files fan1_input,
fan2_input, etc up to the number of configured inputs.

Signed-off-by: Paul Barker <pbarker@konsulko.com>
Link: https://lore.kernel.org/r/20201212195008.6036-3-pbarker@konsulko.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>