board: arm: npcx7/9 EVB: add PECI alias

This commit adds the PECI alias and set the PECI node's status to okay.
With this change, we can build the PECI sample with npcx7m6fb_evb and
npcx9m6f_evb to test the PECI driver.

Signed-off-by: Jun Lin <CHLin56@nuvoton.com>