driver: timer: npcx: add system kernel timer support.

This CL introduces a kernel device driver implemented by the internal
64/32-bit timers in Nuvoton NPCX series. Via these two kinds of timer,
the driver provides an standard "system clock driver" interface.

It includes:
 - A system timer based on an ITIM64 (Internal 64-bit timer) instance,
   clocked by APB2 which freq is CONFIG_SYS_CLOCK_HW_CYCLES_PER_SEC.
 - Its prescaler is set to 1 and provide the kernel cycles reading
   without handling overflow mechanism.
 - A event timer based on an ITIM32 (Internal 32-bit timer) instance,
   clocked by LCLK which frequency is 32KHz and still activated when ec
   entered "idle/deep idle" power state for better power consumption.
 - Its prescaler is set to 1 and provide timeout event mechansim.
 - Compensate system timer which clock is gating for better power
   consumption after ec left"idle/deep idle" power state.

This CL passed starve, timer_api, and timer_monotonic test suites.

Signed-off-by: Mulin Chao <mlchao@nuvoton.com>