arch-riscv: Add bootloader+kernel workload (#390)

Aims to boot OpenSBI + Linux kernel.

src/python/gem5/components/boards/riscv_board.py

@@ -259,6 +259,12 @@ class RiscvBoard(AbstractSystemBoard, KernelDiskWorkload):
             )
             root.append(node)
 
+        node = FdtNode(f"chosen")
+        bootargs = " ".join(self.get_default_kernel_args())
+        node.append(FdtPropertyStrings("bootargs", [bootargs]))
+        node.append(FdtPropertyStrings("stdout-path", ["/uart@10000000"]))
+        root.append(node)
+
         # See Documentation/devicetree/bindings/riscv/cpus.txt for details.
         cpus_node = FdtNode("cpus")
         cpus_state = FdtState(addr_cells=1, size_cells=0)
@@ -432,7 +438,7 @@ class RiscvBoard(AbstractSystemBoard, KernelDiskWorkload):
         uart_node.append(
             FdtPropertyWords("interrupt-parent", soc_state.phandle(plic))
         )
-        uart_node.appendCompatible(["ns8250"])
+        uart_node.appendCompatible(["ns8250", "ns16550a"])
         soc_node.append(uart_node)
 
         # VirtIO MMIO disk node