dts: npcx: arrange device-tree files of npcx7 series.

Arrange device-tree files of npcx7 series by following steps:

1. Move device-tree declarations of npcx family to npcx.dtsi.
2. Leave specific device declarations of npcx7 series to npcx7.dtsi.
3. Move chip series related mapping tables such as
   npcx7-miwus-wui-map.dtsi and so on to npcx/npcx7 folder.
4. Move common device-tree declarations used in each npcx series to
   npcx-miwus-wui-map.dtsi and so on to npcx folder.

Then, the npcx device-tree folders are arranged to:

dts/arm/nuvoton
    |--npcx
    |    |--npcx7
    |    |    |--npcx7-miwus-wui-map.dtsi
    |    |    |--npcx7-alts-map.dtsi
    |    |    |--.....
    |    |--npcx-miwus-wui-map.dtsi
    |    |--npcx-alts-map.dtsi
    |    |--.....
    |
    |--npcx.dtsi
    |--npcx7.dtsi

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