riscv: Add initial support for Shakti C machine

riscv: Add initial support for Shakti C machine

Add support for emulating Shakti reference platform based on C-class
running on arty-100T board.

https://gitlab.com/shaktiproject/cores/shakti-soc/-/blob/master/README.rst

Signed-off-by: Vijai Kumar K <vijai@behindbytes.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20210401181457.73039-3-vijai@behindbytes.com
[Changes by AF:
 - Check for mstate->firmware before loading it
]
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>