PCI: keystone: Add start_link()/stop_link() dw_pcie_ops

Add start_link()/stop_link() dw_pcie_ops and invoke ks_pcie_start_link()
directly from host_init. start_link()/stop_link() ops are required for
adding EP mode support.

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