doc: update QEMU RISC-V documentation

The available defconfigs for RISC-V QEMU have changed. We now have
configurations to compile U-Boot to run in supervisor mode and for
U-Boot SPL. Update the QEMU RISC-V documentation to reflect these
changes.

Signed-off-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>