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: Wesley Terpstra <wesley@sifive.com>
[paul.walmsley@sifive.com: split initial patch into one arch/riscv
 patch and one drivers/pci patch]
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>