treewide: dts: use keycode defines from input dt-binding

treewide: dts: use keycode defines from input dt-binding

All compiled device tree files not mentioned are binary identical to the
former ones.

Fix the obvious decimal/hex confusion for the power key of ramips/M2M.dts.

Due to the include of the input binding header, the BTN_* node names in:

  - ramips/GL-MT300A.dts
  - ramips/GL-MT300N.dts
  - ramips/GL-MT750.dts
  - ramips/Timecloud.dts

will be changed by the compiler to the numerical equivalent.

Move the binding include of lantiq boards to the file where they are
used the first time to hint the user where the values do come from.

Signed-off-by: Mathias Kresin <dev@kresin.me>