PCI: keystone: Cleanup ks_pcie_link_up()

ks_pcie_link_up() uses registers from the designware core to get the
status of the link. Move the register defines to pcie-designware.h
and cleanup ks_pcie_link_up().

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