bmips: add experimental PCI/PCIe support

These PCI drivers are a bit hacky and definitely not suitable for upstreaming,
but hopefully we can use them as a base for developing proper upstream PCI
drivers.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>

target/linux/bmips/files/arch/mips/pci/fixup-bmips.c

@@ -0,0 +1,18 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ *  Copyright (C) 2020 Álvaro Fernández Rojas <noltari@gmail.com>
+ */
+
+#include <linux/pci.h>
+
+int bmips_pci_irq = -1;
+
+int pcibios_plat_dev_init(struct pci_dev *pci_dev)
+{
+	return PCIBIOS_SUCCESSFUL;
+}
+
+int pcibios_map_irq(const struct pci_dev *pci_dev, u8 slot, u8 pin)
+{
+	return bmips_pci_irq;
+}