reset: Add missing function stub for device_reset

The Mediatek's thermal driver fails to compile when the RESET_CONTROLLER
option is not set. Logically, as the driver depends on this option to compile,
the Kconfig should select it but actually that is not correct because the
Kconfig provides also the COMPILE_TEST to increase the compile test coverage.

By providing the missing 'device_reset' stub for the driver in reset.h, that
let the kernel to compile on different platforms with the Mediatek thermal
driver enabled with the COMPILE_TEST option.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>