PCI: keystone: Use uniform function naming convention

No functional change. Some function names begin with ks_dw_pcie_*
and some function names begin with ks_pcie_*. Modify it so that
all function names begin with ks_pcie_*.
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>