hw/riscv: Split out the boot functions

hw/riscv: Split out the boot functions

Split the common RISC-V boot functions into a seperate file. This allows
us to share the common code.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>