clocksource/drivers/arm_global_timer: Make gt_target_rate unsigned long

Change the data type of gt_target_rate to unsigned long as this is what
we get back from clk_get_rate().

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20240225151336.2728533-2-martin.blumenstingl@googlemail.com