thermal: allow spear-thermal driver to be a module

When the thermal subsystem is a loadable module, the spear driver
fails to build:

drivers/thermal/built-in.o: In function `spear_thermal_exit':
spear_thermal.c:(.text+0xf8): undefined reference to `thermal_zone_device_unregister'
drivers/thermal/built-in.o: In function `spear_thermal_probe':
spear_thermal.c:(.text+0x230): undefined reference to `thermal_zone_device_register'

This changes the symbol to a tristate, so Kconfig can track the
dependency correctly.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>