Add pwm id as fan pwm target
Add pwm id as fan pwm target

The current code assumes fan and pwm has the same id.

This commit add the support to use pwmX as fan pwm target by
PWM_TARGET_ environment.

E.g.
PWM_TARGET_fan0 = "1"
PWM_TARGET_fan1 = "1"
PWM_TARGET_fan2 = "2"
PWM_TARGET_fan3 = "2"

The above config tells hwmon to set pwm1 for fan0/1's pwm target,
and set pwm2 for fan2/3's pwm target.

Change-Id: Ic1d850ca3d6d20274d905fa9004d7eba2718e9eb
Signed-off-by: Lei YU <a3094571254a8e1f82a212e5bbbfd045adf48936@gmail.com>

openbmc/phosphor-hwmon,openbmc/phosphor-hwmon