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 <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>