hwmon: (adt7470) Expose PWM frequency to sysfs

The ADT7470 supports a variety of PWM frequencies. This patch allows the
frequency to be configured and viewed through the sysfs entry pwm1_freq.

Signed-off-by: Joshua Scott <joshua.scott@alliedtelesis.co.nz>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>