dt-bindings: embedded-controller: add Traverse Ten64 board controller

dt-bindings: embedded-controller: add Traverse Ten64 board controller

Add device tree binding for the board (micro)controller on Ten64 family
boards[1].

The schema is simple and is (presently) only consumed by U-Boot, but it
is possible nvmem, watchdog and other features could be described in
the future, as well as extension to future Traverse boards.

[1] https://ten64doc.traverse.com.au/hardware/microcontroller/

Signed-off-by: Mathew McBride <matt@traverse.com.au>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>