update OpenSBI to v0.4 for QEMU virt machine

Author: wangrunji0408

tools/opensbi/README.md

@@ -1,9 +1,9 @@
 # OpenSBI
 
-These are binary release of OpenSBI on this [commit](https://github.com/riscv/opensbi/tree/194dbbe5a13dff2255411c26d249f3ad4ef42c0b) at 2019.04.15.
+These are binary release of OpenSBI v0.4.
 
-- virt_rv32.elf: opensbi-0.3-rv32-bin/platform/qemu/virt/firmware/fw_jump.elf
-- virt_rv64.elf: opensbi-0.3-rv64-bin/platform/qemu/virt/firmware/fw_jump.elf
+- virt_rv32.elf: opensbi-0.4-rv32-bin/platform/qemu/virt/firmware/fw_jump.elf
+- virt_rv64.elf: opensbi-0.4-rv64-bin/platform/qemu/virt/firmware/fw_jump.elf
 
 NOTE: The [official v0.3 release](https://github.com/riscv/opensbi/releases/tag/v0.3) has bug on serial interrupt. Also, Rocket-Chip based CPUs (including SiFive Unleashed) seem to have unintended behavior on