clocksource/drivers/vf-pit: Allocate the struct timer at init time

clocksource/drivers/vf-pit: Allocate the struct timer at init time

Instead of having a static global structure for a timer, let's
allocate it dynamically so we can create multiple instances in the
future to support multiple timers. At the same time, add the
rollbacking code in case of error.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20250804152344.1109310-8-daniel.lezcano@linaro.org