RISC-V: Initialize SBI early

Currently, SBI is initialized towards the end of arch setup. This prevents
the set memory operations to be invoked earlier as it requires a full tlb
flush.

Initialize SBI as early as possible.

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>