drivers: pcie: initialize after interrupt controllers

PCIe devices refer to interrupt nodes, but are initialized with the same
priority, making the sequence depending on the linking order. Add a new
symbol and set it to one unit after intc to ensure that the
initialization sequence is stable.

Found with:

west build -p -b qemu_cortex_a53 \
	samples/drivers/virtualization/ivshmem/doorbell \
	-DCONFIG_CHECK_INIT_PRIORITIES=y
...
ERROR: /soc/pcie@4010000000 PRE_KERNEL_1 40 <
	/soc/interrupt-controller@8000000/its@8080000 POST_KERNEL 40

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>