PCI: keystone: Use SYSCON APIs to get device ID from control module

Control module registers should be read using syscon APIs.
pci-keystone.c uses platform_get_resource() to get control module registers.
Fix it here by using syscon APIs to get device id from control module.

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