riscv: Generate PCI host DT node

This patch extends FDT generation to generate PCI host DT node.

Of course, PCI host for Guest/VM is not useful at the moment
because it's mostly for PCI pass-through and we don't have
IOMMU and interrupt routing available for KVM RISC-V. In future,
we might be able to use PCI host for VirtIO PCI transport or
other software emulated PCI devices.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Link: https://lore.kernel.org/r/20211119124515.89439-9-anup.patel@wdc.com


Signed-off-by: Will Deacon <will@kernel.org>