PCI: keystone: Move resources initialization to prepare for EP support

Move platform_get_resource() calls for resources that are applicable to
both host and endpoint mode (ie "dbics" and "app") from
ks_add_pcie_port() to the probe() callback, in preparation for adding
endpoint support to pci-keystone driver.

No functional change intended.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>