ARM: dts: aspeed: Initial device tree for AMD EthanolX

Initial introduction of AMD EthanolX platform equipped with an
Aspeed ast2500 BMC manufactured by AMD.

AMD EthanolX platform is an AMD customer reference board with an
Aspeed ast2500 BMC manufactured by AMD.

This adds AMD EthanolX device tree file including the flash layout
used by EthanolX BMC machines.

Signed-off-by: Supreeth Venkatesh <supreeth.venkatesh@amd.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>