riscv: include generic support for MSI irqdomains

Some RISC-V systems include PCIe host controllers that support PCIe
message-signaled interrupts.  For this to work on Linux, we need to
enable PCI_MSI_IRQ_DOMAIN and define struct msi_alloc_info.  Support
for the latter is enabled by including the architecture-generic msi.h
include.
Signed-off-by: NWesley Terpstra <wesley@sifive.com>
[paul.walmsley@sifive.com: split initial patch into one arch/riscv
 patch and one drivers/pci patch]
Signed-off-by: NPaul Walmsley <paul.walmsley@sifive.com>

arch/riscv/include/asm/Kbuild

@@ -22,6 +22,7 @@ generic-y += kvm_para.h
 generic-y += local.h
 generic-y += local64.h
 generic-y += mm-arch-hooks.h
+generic-y += msi.h
 generic-y += percpu.h
 generic-y += preempt.h
 generic-y += sections.h