clocksource/drivers/atmel_tcb: Fix kconfig dependency

clocksource/drivers/atmel_tcb: Fix kconfig dependency

Build-testing this driver on arm without CONFIG_OF produces a warning:

drivers/clocksource/timer-atmel-tcb.c:368:34: error: 'atmel_tcb_of_match' defined but not used [-Werror=unused-const-variable=]
  368 | static const struct of_device_id atmel_tcb_of_match[] = {
      |                                  ^~~~~~~~~~~~~~~~~~

Change the dependency to require CONFIG_OF for build testing to
avoid the warning. Testing remains possible on all architectures
as CONFIG_OF is user-selectable.

Cc: Nicolas Ferre <nicolas.ferre@microchip.com>
Cc: Claudiu Beznea <claudiu.beznea@tuxon.dev>
Reviewed-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20250409155313.1096915-1-arnd@kernel.org
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>