dt-bindings: ipmi: Convert IPMI-SMIC bindings to json-schema

Convert the generic IPMI controller bindings to DT schema format
using json-schema.

The device_type property is deprecated for most node per the DT spec,
but at least the Linux driver matches on it, so I keep it in.

The Linux driver parses for some additional and optional properties,
but there are no in-tree users. Let's allow extra properties to cover
any other users.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>