hwmon: (adt7475) add high frequency support

Systems using 4-wire fans usually require high frequency (22.5kHz)
output on the pwm. Add 22500 as a valid option in the pwmfreq_table. In
high frequency mode the low-order bit are ignored so they can safely be
set to 0.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>