x86: Clean up the board dts files

x86: Clean up the board dts files

This commits cleans up the board dts files.

- Correct the serial port register size to 8
- Remove the misleading status = "disabled" statement in the
  serial.dtsi
- Move the inclusion of skeleton.dtsi from serial.dtsi to board
  dts files
- Let the board dts file define stdout-path in the chosen node
- Remove device nodes in board dts files thar are duplicated to
  skeleton.dtsi

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>