ARM: dts: aspeed: Add Facebook Wedge40 BMC

Add initial version of device tree for Facebook Wedge40 AST2400 BMC
platform.

Signed-off-by: Tao Ren <taoren@fb.com>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Joel Stanley <joel@jms.id.au>