ARM: dts: aspeed: Add AST2600 SoC support

AST2600 is the 7th generation of Aspeed SoC designated for
Interated Remote Management Processor.

AST2600 has significant performance improvement by integrating
1.2GHz dual-core ARM Cortex A7 (r0p5) CPU with FPU. Most of the
controllers are also improved with more features and better
performance than preceding AST24xx/AST25xx.

Signed-off-by: Chia-Wei, Wang <chiawei_wang@aspeedtech.com>
Reviewed-by: Ryan Chen <ryan_chen@aspeedtech.com>