ARM: dts: aspeed: Add Portwell Neptune machine

Initial introduction of Portwell Neptune family equipped with
Aspeed 2500 BMC SoC. Neptune is a x86 server development kit with a
ASPEED ast2500 BMC manufactured by Portwell. Specifically, This
adds the neptune platform device tree file including the flash
layout used by the neptune machines.

Signed-off-by: Amithash Prasad <amithash@fb.com>
Acked-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Joel Stanley <joel@jms.id.au>