clocksource/drivers/timer-microchip-pit64b: Remove mmio selection

PIT64B timer driver doesn't depend on CLKSRC_MMIO since
commit e85c1d21 ("clocksource/drivers/timer-microchip-pit64b:
Add clocksource suspend/resume"). Remove the selection.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20220304133601.2404086-2-claudiu.beznea@microchip.com


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