This adds a function to determine the 8259 level/edge mask word.
With this function we can now (in theory) support any IBM PReP
machine.  We therefore add a little bit more code to fallback
to using residual data if we don't recognise the machine.
Signed-off-by: Leigh Brown <leigh@solinno.co.uk>
Signed-off-by: Tom Rini <trini@kernel.crashing.org>