x86: Use the IRQ device when setting up the mptable

Instead of searching for the device tree node, use the IRQ device which has
a record of it.

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