drivers: hwmon: Support for ASPEED PWM/Fan tach

The ASPEED AST2400/2500 PWM controller supports 8 PWM output ports.
The ASPEED AST2400/2500 Fan tach controller supports 16 tachometer
inputs.
The device driver matches on the device tree node. The configuration
values are read from the device tree and written to the respective
registers.
The driver provides a sysfs entries through which the user can
configure the duty-cycle value (ranging from 0 to 100 percent) and read
the fan tach rpm value.

Signed-off-by: Jaghathiswari Rankappagounder Natarajan <jaghu@google.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>