PCI: cadence: Refactor driver to use as a core library

Cadence PCIe host and endpoint IP may be embedded into a variety of
SoCs/platforms. Let's extract the platform related APIs/Structures in the
current driver to a separate file (pcie-cadence-plat.c), such that the
common functionality can be used by future platforms.

Signed-off-by: Tom Joseph <tjoseph@cadence.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Andrew Murray <andrew.murray@arm.com>