RISC-V: Move dynamic relocation section under __init

Dynamic relocation section are only required during boot. Those sections
can be freed after init. Thus, it can be moved to __init section.

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Tested-by: Greentime Hu <greentime.hu@sifive.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>