thermal: db8500_cpufreq_cooling: Compile with COMPILE_TEST

This driver only has runtime but no build time dependencies, so it can be
built for testing purposes if the Kconfig COMPILE_TEST option is enabled.

This is useful to have more build coverage and make sure that drivers are
not affected by changes that could cause build regressions.

Signed-off-by: Luis de Bethencourt <luisbg@osg.samsung.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>