riscv: ensure cpu_ops_sbi is declared

riscv: ensure cpu_ops_sbi is declared

Sparse complains that cpu_ops_sbi is used undeclared:
arch/riscv/kernel/cpu_ops_sbi.c:17:29: warning: symbol 'cpu_ops_sbi' was not declared. Should it be static?

Fix the warning by adding cpu_ops_sbi to cpu_ops_sbi.h & including that
where used.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20220714080235.3853374-1-conor.dooley@microchip.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>