Use Keystone specific msi_irq_chip to configure the MSI controller
logic in the PCIe keystone wrapper instead of using the default
Designware msi_irq chip (dw_pci_msi_bottom_irq_chip) with
callback functions for configuring the Keystone MSI controller.
This will help to remove Keystone specific callback functions
added in dw_pcie_host_ops.

Move the default msi_irq_chip assignment to dw_pcie_host_init since
platforms that doesn't use the default msi_irq_chip will assign
msi_irq_chip in the msi_host_init() callback.
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>