hw/core/or-irq: Support more than 16 inputs to an OR gate

hw/core/or-irq: Support more than 16 inputs to an OR gate

For the IoTKit MPC support, we need to wire together the
interrupt outputs of 17 MPCs; this exceeds the current
value of MAX_OR_LINES. Increase MAX_OR_LINES to 32 (which
should be enough for anyone).

The tricky part is retaining the migration compatibility for
existing OR gates; we add a subsection which is only used
for larger OR gates, and define it such that we can freely
increase MAX_OR_LINES in future (or even move to a dynamically
allocated levels[] array without an upper size limit) without
breaking compatibility.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20180604152941.20374-10-peter.maydell@linaro.org