PCI: dwc: Add support to use non default msi_irq_chip

Platforms using DesignWare IP use dw_pci_msi_bottom_irq_chip for
configuring the MSI controller logic within the DesignWare IP. However
certain platforms like Keystone (K2G) which uses DesignWare IP have
their own MSI controller logic. For handling such platforms,
the irqchip ops use msi_irq_ack(), msi_set_irq(), msi_clear_irq()
callback functions.

Add support to use different msi_irq_chip with default set to
dw_pci_msi_bottom_irq_chip. This is in preparation to get rid of
msi_irq_ack(), msi_set_irq(), msi_clear_irq() and other Keystone
specific dw_pcie_host_ops.

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