x86: irq: Get irq_router's bdf via dm_pci_get_bdf()

There is no need to parse PCH's <reg> property as we have already
a DM PCI API dm_pci_get_bdf() that can handle this.

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