PCI: dwc: Rework MSI initialization

PCI: dwc: Rework MSI initialization

There are 3 possible MSI implementations for the DWC host. The first is
using the built-in DWC MSI controller. The 2nd is a custom MSI
controller as part of the PCI host (keystone only). The 3rd is an
external MSI controller (typically GICv3 ITS). Currently, the last 2
are distinguished with a .msi_host_init() hook with the 3rd option using
an empty function. However we can detect the 3rd case with the presence
of 'msi-parent' or 'msi-map' properties, so let's do that instead and
remove the empty functions.

Link: https://lore.kernel.org/r/20201105211159.1814485-10-robh@kernel.org
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Jingoo Han <jingoohan1@gmail.com>
Cc: Murali Karicheri <m-karicheri2@ti.com>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Minghuan Lian <minghuan.Lian@nxp.com>
Cc: Mingkai Hu <mingkai.hu@nxp.com>
Cc: Roy Zang <roy.zang@nxp.com>
Cc: Gustavo Pimentel <gustavo.pimentel@synopsys.com>
Cc: linuxppc-dev@lists.ozlabs.org