hw/m68k/next-cube: Make next_irq take NeXTPC* as its opaque

hw/m68k/next-cube: Make next_irq take NeXTPC* as its opaque

Make the next_irq function take a NeXTPC* as its opaque rather than
the M68kCPU*.  This will make it simpler to turn the next_irq
function into a gpio input line of the NeXTPC device in the next
commit.

For this to work we have to pass the CPU to the NeXTPC device via a
link property, in the same way we do in q800.c (and for the same
reason).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20210115201206.17347-6-peter.maydell@linaro.org>
Signed-off-by: Thomas Huth <huth@tuxfamily.org>