PCI: Regroup all PCI related entries into drivers/pci/Makefile

Clean up drivers/Makefile by moving the pci/endpoint and pci/dwc entries
from drivers/Makefile into drivers/pci/Makefile.

Since we don't want to introduce any dependency between CONFIG_PCI and
CONFIG_PCI_ENDPOINT, we now always execute drivers/pci/Makefile.

Hence all Makefiles in drivers/pci/ were updated accordingly so no file is
compiled when CONFIG_PCI is not defined.

Also, we add a comment to reinforce that EPC and EPF libraries must be
initialized before their users. Hence built-in EPC drivers, such as
those of Designware, are linked after the endpoint core libraries.

Finally, we add another comment to explain why obj-y has been chosen
instead of obj-$(CONFIG_PCIE_DW) to parse the dwc/ sub-folder.

Signed-off-by: Cyrille Pitchen <cyrille.pitchen@free-electrons.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>