doc: usage: Update vxworks doc to mention RISC-V support

At present the doc only mentions Arm, PowerPC and x86. RISC-V support
has been added since VxWorks SR0650 support for a while, and U-Boot
supports loading a RISC-V VxWorks kernel too. Let's document it.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>