dt-bindings: aspeed: Add silicon id node to SCU

Different ASPEED families have various unique hardware silicon
identifiers within the SoC.

Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://lore.kernel.org/r/20200921091644.133107-2-joel@jms.id.au


Signed-off-by: Joel Stanley <joel@jms.id.au>