dt-bindings: aspeed: Add Secure Boot Controller bindings

The secure boot controller was first introduced in the AST2600.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211117035106.321454-2-joel@jms.id.au


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