RISC-V: Declare per cpu boot data as static

The per cpu boot data is only used within the cpu_ops_sbi.c. It can
be delcared as static.

Fixes: 9a2451f1 ("RISC-V: Avoid using per cpu array for ordered booting")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Atish Patra <atishp@rivosinc.com>
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>