Re-organize modules of `loader` to follow image format, enable reuse of
code across different architectures. Its interface is unchanged.

Signed-off-by: Ruoqing He <heruoqing@iscas.ac.cn>

Code in `fdt.rs` do not contain architecture specific code as of
`aarch64`, it's only used by `aarch64`, move it one level up to allow
reusing on riscv64 platform.

Signed-off-by: Ruoqing He <heruoqing@iscas.ac.cn>

Code in `mod.rs` of `aarch64` do not contain architecture specific code
as of `aarch64`, it's only used by `aarch64` to run benchmark tests,
move it one level up and rename it to `fdt.rs` to allow reusing on
riscv64 platform.

Signed-off-by: Ruoqing He <heruoqing@iscas.ac.cn>