dm: x86: Create a driver for x86 interrupts

It seems likely that at some point we will want a generic interrupt uclass.
But this is a big undertaking as it involves unifying code across multiple
architectures.

As a first step, create a simple IRQ uclass and a driver for x86. This can
be generalised later as required.

Adjust pirq_init() to probe this driver, which has the effect of creating
routing tables and setting up the interrupt routing. This is a start
towards making interrupts fit better with driver model.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>