clocksource/drivers/vf-pit: Register the clocksource from the driver

clocksource/drivers/vf-pit: Register the clocksource from the driver

The function clocksource_mmio_init() uses its own global static
clocksource variable making no possible to have several instances of a
clocksource using this function. In order to support that, let's add
the clocksource structure to the pit structure and use the
clocksource_register_hz() function instead.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20250804152344.1109310-10-daniel.lezcano@linaro.org