thermal: allow u8500-thermal driver to be a module

When the thermal subsystem is a loadable module, the u8500 driver
fails to build:

drivers/thermal/built-in.o: In function `db8500_thermal_probe':
db8500_thermal.c:(.text+0x96c): undefined reference to `thermal_zone_device_register'
drivers/thermal/built-in.o: In function `db8500_thermal_work':
db8500_thermal.c:(.text+0xab4): undefined reference to `thermal_zone_device_update'

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>