dt-bindings: misc: achc: Add device tree binding for GE ACHC

Add Device Tree binding document for GE Healthcare USB Management
Controller (ACHC).

Signed-off-by: Martyn Welch <martyn.welch@collabora.co.uk>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>