doc: qemu-riscv: Fix opensbi build instructions

Latest opensbi uses generic platform for Qemu. Update the build
instructions.

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Rick Chen <rick@andestech.com>
1 file changed