driver: sensor: npcx: add tachometer sensor support.

In NPCX7 series, it contains two tachometer (TACH) modules that contains
two Independent timers (counter 1 and 2). They are used to capture a
counter value when an event is detected via the external pads (TA or
TB).

The CL also includes:
— Add npcx tachometer device tree declarations.
— Zephyr sensor api implementation for tachometer.
— Enable "tach1" device in npcx7m6fb.dts for testing.

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