mvebu and aardvark HW have PCIe capabilities on different offset in PCI
config space. Extend pci-bridge-emul.c code to allow setting custom driver
custom value where PCIe capabilities starts.

With this change PCIe capabilities of both drivers are reported at the same
location as where they are reported by U-Boot - in their real HW offset.

Link: https://lore.kernel.org/r/20220824112124.21675-1-pali@kernel.orgSigned-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>