ARM: dts: aspeed: Add secure boot controller node

The ast2600 has a secure boot controller.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Ryan Chen <ryan_chen@aspeedtech.com>
Link: https://lore.kernel.org/r/20211117035106.321454-3-joel@jms.id.au


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