hwmon: (scpi) add thermal-of dependency

The newly added scpi thermal support is broken when the scpi driver
is built-in but the thermal driver is a loadable module:

drivers/built-in.o: In function `scpi_hwmon_probe':
(.text+0x444d70): undefined reference to `thermal_zone_of_sensor_unregister'
(.text+0x444d94): undefined reference to `thermal_zone_of_sensor_register'
drivers/built-in.o: In function `scpi_hwmon_remove':
(text+0x444e6c): undefined reference to `thermal_zone_of_sensor_unregister'

This uses the same Kconfig trick that we have in a couple of other
drivers already to ensure we can only select the driver in valid
configurations when either THERMAL_OF is disabled, or when with a
dependency on CONFIG_THERMAL that can force SCPI to be a loadable
module in the case I was hitting.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 68acc77a ("hwmon: Support thermal zones registration for SCP temperature sensors")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>