arm64: dts: ten64: add board controller binding

arm64: dts: ten64: add board controller binding

The board (micro)controller[1] is responsible for functions such as
power supply sequencing, SoC reset as well as serial/MAC address storage,
bootcount and scratch registers.

There is currently no Linux kernel driver for this controller, however,
there is a driver in U-Boot which utilises this binding.

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

Signed-off-by: Mathew McBride <matt@traverse.com.au>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>