roms: Add OpenSBI version 0.4

Add OpenSBI version 0.4 as a git submodule and as a prebult binary.

OpenSBI (https://github.com/riscv/opensbi) aims to provide an open-source
reference implementation of the RISC-V Supervisor Binary Interface (SBI)
specifications for platform-specific firmwares executing in M-mode. For all
supported platforms, OpenSBI provides several runtime firmware examples.
These example firmwares can be used to replace the legacy riscv-pk bootloader
and enable the use of well-known bootloaders such as U-Boot.

OpenSBI is distributed under the terms of the BSD 2-clause license
("Simplified BSD License" or "FreeBSD License", SPDX: BSD-2-Clause). OpenSBI
source code also contains code reused from other projects desribed here:
https://github.com/riscv/opensbi/blob/master/ThirdPartyNotices.md.

In this case all of the code we are using from OpenSBI is BSD 2-clause
as we aren't using the Kendryte code (Apache-2.0) with QEMU and libfdt
is dual licensed as BSD 2-clause (and GPL-2.0+). OpenSBI isn't being
linked with QEMU either it is just being included with QEMU.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>

pc-bios/README

@@ -63,3 +63,14 @@
   ARM. Licensing information is given in "edk2-licenses.txt". The image files
   are described by the JSON documents in the "pc-bios/descriptors" directory,
   which conform to the "docs/interop/firmware.json" schema.
+
+- OpenSBI (https://github.com/riscv/opensbi) aims to provide an open-source
+  reference implementation of the RISC-V Supervisor Binary Interface (SBI)
+  specifications for platform-specific firmwares executing in M-mode. For all
+  supported platforms, OpenSBI provides several runtime firmware examples.
+  These example firmwares can be used to replace the legacy riscv-pk bootloader
+  and enable the use of well-known bootloaders such as U-Boot.
+  OpenSBI is distributed under the terms of the BSD 2-clause license
+  ("Simplified BSD License" or "FreeBSD License", SPDX: BSD-2-Clause). OpenSBI
+  source code also contains code reused from other projects desribed here:
+  https://github.com/riscv/opensbi/blob/master/ThirdPartyNotices.md.