dts: misc: npcx: add new DT node for booter variant

The booter (bootloader) behavior may be different in different NPCX
chip series. One example is that the booter sets host interface type in
NPCX7 series but leave the firmware to set it in NPCX9 series.
This commit adds a new DT node to record variants in its properties.
NPCX drivers can understand if they need to configure the related
setting by checking the node's properties.

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