Input: sun4i-ts - add thermal driver dependency

The sun4i-ts driver has had a dependency on the thermal code
with the addition of the thermal zone sensor support, but this
is not currently enforced in Kconfig, so with TOUCHSCREEN_SUN4I=y,
THERMAL=m and THERMAL_OF=y we get

drivers/built-in.o: In function `sun4i_ts_remove':
:(.text+0x2376f4): undefined reference to `thermal_zone_of_sensor_unregister'
drivers/built-in.o: In function `sun4i_ts_probe':
:(.text+0x237a94): undefined reference to `thermal_zone_of_sensor_register'
:(.text+0x237c00): undefined reference to `thermal_zone_of_sensor_unregister'

We need the dependency on THERMAL in order to ensure that this
driver becomes a loadable module if the thermal support itself
is modular, while the dependency on THERMAL_OF is a runtime
dependency and the driver will still build if it is missing.
It is entirely possible to build sun4i-ts without THERMAL_OF
just to use the hwmon sensors and/or touchscreen.

Fixes: 22369710 ("Input: sun4i-ts - add thermal zone sensor support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
[wens@csie.org: Fix description and Kconfig dependencies]
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>