hwmon: (aspeed-pwm) add THERMAL dependency

With CONFIG_THERMAL=m, a built-in aspeed pwm tacho driver causes
a link error:

drivers/hwmon/aspeed-pwm-tacho.o: In function `aspeed_pwm_tacho_probe':
aspeed-pwm-tacho.c:(.text+0x7f0): undefined reference to `thermal_of_cooling_device_register'

This adds a dependency similar to what other hwmon drivers use,
ensuring that the aspeed driver cannot be built-in in this
case but has to be a module. With THERMAL=n, we still allow building it.

Fixes: 2d7a548a ("drivers: hwmon: Support for ASPEED PWM/Fan tach")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>