ARM: dts: aspeed: Add device tree for Ampere's Mt. Mitchell BMC

The Mt. Mitchell BMC is an ASPEED AST2600-based BMC for the Mt. Mitchell
hardware reference platform with AmpereOne(TM) processor.

Signed-off-by: Quan Nguyen <quan@os.amperecomputing.com>
Signed-off-by: Phong Vo <phong@os.amperecomputing.com>
Signed-off-by: Thang Q. Nguyen <thang@os.amperecomputing.com>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Link: https://lore.kernel.org/r/20220817071539.176110-3-quan@os.amperecomputing.com


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