dt-bindings: arm: atmel: Move various sys registers out of SoC binding doc

In preparation to convert board-level bindings to json-schema, move
various misc SoC bindings out to their own file.

Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Nicolas Ferre <nicolas.ferre@microchip.com>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: devicetree@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Rob Herring <robh@kernel.org>