riscv: Extend board compatible string with "qemu,mbv"

Extend compatible string to match the latest change in dt binding.

Fixes: 7576ab2f ("riscv: Add support for AMD/Xilinx MicroBlaze V")
Signed-off-by: Michal Simek <michal.simek@amd.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>