clocksource/drivers/tcb_clksrc: Move Kconfig option

Move the ATMEL_TCB_CLKSRC option to drivers/clocksource and make it silent
if COMPILE_TEST is not selected.

Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>