dt-bindings: arm: cpus: Add ASPEED SMP

The AST2600 SoC contains two CPUs and requires the operating system to
bring the second one out of firmware.

Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Joel Stanley <joel@jms.id.au>