ARM: dts: aspeed: Add Lenovo Hr855xg2 BMC

Initial introduction of Lenovo Hr855xg2 family equipped with
Aspeed 2500 BMC SoC. Hr855xg2 is a x86 server development kit
with a ASPEED ast2500 BMC manufactured by Lenovo.

This adds the Hr855xg2 platform device tree file used by the Hr855xg2
BMC machines.

Signed-off-by: Andrew Peng <pengms1@lenovo.com>
Signed-off-by: Yonghui Liu <liuyh21@lenovo.com>
Signed-off-by: Lisa Liu <liuyj19@lenovo.com>
Signed-off-by: Harry Sung <hsung1@lenovo.com>
Signed-off-by: Derek Lin <dlin23@lenovo.com>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Joel Stanley <joel@jms.id.au>