PCI: keystone: Merge pci-keystone-dw.c and pci-keystone.c

No functional change. Having two different files for keystone PCI driver
doesn't serve any purpose. Merge pci-keystone-dw.c and pci-keystone.c
into a single pci-keystone.c file and remove pci-keystone.h.

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