PCI/MSI: Enable PCI_MSI_IRQ_DOMAIN support for RISC-V

Add RISC-V as an arch that supports PCI_MSI_IRQ_DOMAIN.  The related change
to generate asm/msi.h is 251a4488 ("riscv: include generic support for
MSI irqdomains").

Link: https://lore.kernel.org/r/alpine.DEB.2.21.9999.1907251426450.32766@viisi.sifive.com


Signed-off-by: Wesley Terpstra <wesley@sifive.com>
[paul.walmsley@sifive.com: wrote patch description; split this
 patch from the arch/riscv patch]
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>