boards: arm: npcx7m6fb_evb: An example for low-voltage feature.

This CL provided an example of how turns on the low-voltage level
detection feature in npcx series. It demonstrates enabling low-voltage
level detection of I2C1_0 SCL/SDA io-pads if the power rail of their PUs
is 1.8V.

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