qemu_irq_split() callers should use TYPE_SPLIT_IRQ device instead

Currently QEMU has two ways to say "take this irq line and split it so I can feed it into two devices" -- the old-style function qemu_irq_split(), and the newer TYPE_SPLIT_IRQ QOM device. For new code we prefer the QOM device.

The old qemu_irq_split() function isn't used in many places:

• the realview boards (2 calls)
• the stellaris board (1 call)
• the openrisc_sim board (1 call)
• the exynos4210 gic device (3 calls)
• the exynos4210 combiner device (7 calls)

It would be a nice cleanup to update these five files to use TYPE_SPLIT_IRQ instead, so we can remove the old API.

I'm tentatively marking this as a bitesized task. Notes on that:

• if you're trying this, I suggest starting with the realview board; send a patch for just that to check you have the right idea before tackling the others
• you'll need (or will learn) some basic knowledge about how to create and wire up QOM device objects both in board models and inside other device objects