dt-bindings: ipmi: Convert ASPEED KCS binding to schema

Given the deprecated binding, improve the ability to detect issues in
the platform devicetrees. Further, a subsequent patch will introduce a
new interrupts property for specifying SerIRQ behaviour, so convert
before we do any further additions.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Zev Weiss <zweiss@equinix.com>
Message-Id: <20210608104757.582199-13-andrew@aj.id.au>
Signed-off-by: Corey Minyard <cminyard@mvista.com>