riscv: kexec: Cleanup riscv_kexec_relocate

For readability and simplicity, cleanup the riscv_kexec_relocate code:

- Re-sort the first 4 `mv` instructions against `riscv_kexec_method()`
- Eliminate registers for debugging (s9,s10,s11) and storing const-value (s5,s6)
- Replace `jalr` with `jr` for no-link jump

I tested this on Qemu virt machine and works as it was.

Signed-off-by: Song Shuai <songshuaishuai@tinylab.org>
Link: https://lore.kernel.org/r/20230907103304.590739-2-songshuaishuai@tinylab.org


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>