clocksource/drivers/arm_global_timer: Fix maximum prescaler value

The prescaler in the "Global Timer Control Register bit assignments" is
documented to use bits [15:8], which means that the maximum prescaler
register value is 0xff.

Fixes: 171b45a4 ("clocksource/drivers/arm_global_timer: Implement rate compensation whenever source clock changes")
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20240218174138.1942418-2-martin.blumenstingl@googlemail.com