dts/arm: npcx: move def_lvol_io_list node from board dts to dtsi file.

This CL moves def_lvol_io_list device-tree node from npcx7m6fb_evb.dts
to npcx7m6fb.dtsi. The benefit of it is that we needn't add
def_lvol_io_list node for each board dts file if there are no 1.8V
io-pads on the platform. If so, add them in the specific board dts file
directly.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>
Signed-off-by: Mulin Chao <mlchao@nuvoton.com>