dm: x86: Set up interrupt routing from interrupt_init()

At present interrupt routing is set up from arch_misc_init(). We can do it
a little later instead, in interrupt_init().

This removes the manual pirq_init() call. Where the platform does not have
an interrupt router defined in its device tree, no error is generated. Some
platforms do not have this.

Drop pirq_init() since it is no-longer used.

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