ARM: dts: aspeed: Common dtsi for Facebook AST2500 Network BMCs

This common descirption is included by all Facebook AST2500 Network BMC
platforms to minimize duplicated device entries across Facebook Network
BMC device trees.

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