soc: nuvoton: npcx: update default SYS_CLOCK_HW_CYCLES_PER_SEC

Added support for deriving `SYS_CLOCK_HW_CYCLES_PER_SEC` from the
Device Tree by reading the `clock-frequency` property in the `itim` node.

Signed-off-by: Alvis Sun <yfsun@nuvoton.com>