PCI: keystone: Perform host initialization in a single function

No functional change. Instead of having two functions
ks_pcie_add_pcie_port() and ks_pcie_dw_host_init() for initializing
host, have a single function to perform all the host initialization.

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