PCI: keystone: Cleanup configuration space access

Cleanup configuration space access by removing ks_pcie_cfg_setup()
which has an unncessary check of "if (bus == 0)" which will never be the
case of *_other_conf() and adding macros for configuring the CFG_SETUP
register required for accessing the configuration space of the device.

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