PCI: mvebu: Check that PCI bridge specified in DT has function number zero

Driver cannot handle PCI bridges at non-zero function address. So add
appropriate check. Currently all in-tree kernel DTS files set PCI bridge
function to zero.

Link: https://lore.kernel.org/r/20211125124605.25915-4-pali@kernel.org


Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>