riscv: import the supervisor binary interface header file

riscv: import the supervisor binary interface header file

Import the supervisor binary interface (SBI) header file from Linux
(arch/riscv/include/asm/sbi.h). The last change to it was in commit
6d60b6ee0c97 ("RISC-V: Device, timer, IRQs, and the SBI").

Signed-off-by: Lukas Auer <[email protected]>
Reviewed-by: Anup Patel <[email protected]>
Reviewed-by: Bin Meng <[email protected]>
Reviewed-by: Atish Patra <[email protected]>