ARM: dts: aspeed: Change KCS nodes to v2 binding

Fixes the following warnings for both g5 and g6 SoCs:

    arch/arm/boot/dts/aspeed-g5.dtsi:376.19-381.8: Warning
    (unit_address_vs_reg): /ahb/apb/lpc@1e789000/lpc-bmc@0/kcs1@0: node
    has a unit name, but no reg property

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Joel Stanley <joel@jms.id.au>