hw/timer/npcm7xx_timer: Use new clock_ns_to_ticks()

Use the new clock_ns_to_ticks() function in npcm7xx_timer where
appropriate.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Luc Michel <luc@lmichel.fr>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Hao Wu <wuhaotsh@google.com>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20210219144617.4782-5-peter.maydell@linaro.org