clocksource/drivers/timer-microchip-pit64b: Select driver only on ARM

Microchip PIT64B is currently available on ARM based devices. Thus
select it only for ARM. This allows implementing delay timer.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20230203130537.1921608-2-claudiu.beznea@microchip.com


Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>