iio: adc: sun4i: add THERMAL dependency

With CONFIG_THERMAL=m and CONFIG_SUN4I_GPADC=y, we get a link error
from calling devm_thermal_zone_of_sensor_register:

drivers/iio/built-in.o: In function `sun4i_gpadc_probe':
:(.text+0x1c284): undefined reference to `devm_thermal_zone_of_sensor_register'

This adds a Kconfig dependency to ensure we can only have the ADC
driver as built-in when this function is also built-in, or when
we see the empty stub implementation. When the thermal code is
a module, we can still build the adc driver as a module, too.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: d1caa990 ("iio: adc: add support for Allwinner SoCs ADC")
Signed-off-by: Jonathan Cameron <jic23@kernel.org>