PCI: aardvark: Introduce an advk_pcie_valid_device() helper

In other to mimic other PCIe host controller drivers, introduce an
advk_pcie_valid_device() helper, used in the configuration read/write
functions.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
[lorenzo.pieralisi@arm.com: updated host->controller dir move]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>