Reduce the fan_tach period such that the fan controller uses a shorter
period to measure the rpm.

The original period of 0x1000 was chosen as a conversative value from the
reference implementation.  Through experimentation on the quanta-q71l
board, I was able to drive the number down which ultimately reduced the
time the controller would use to determine the fan_tach.  This value was
recently tested and accepted downstream on the IBM Zaius board which uses
the ast2500.

Future work: It may be worthwhile as this is a tunable parameter to the
system, to allow overriding it through the device tree.

Testing: Tested on an ast2400 sitting on a quanta-q71l and ast2500 on
power9.
Signed-off-by: Patrick Venture <venture@google.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>