riscv: poison SBI calls for M-mode

There is no SBI when we run in M-mode, so fail the compile for any code
trying to use SBI calls.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>

arch/riscv/include/asm/sbi.h

@@ -8,6 +8,7 @@
 
 #include <linux/types.h>
 
+#ifdef CONFIG_RISCV_SBI
 #define SBI_SET_TIMER 0
 #define SBI_CONSOLE_PUTCHAR 1
 #define SBI_CONSOLE_GETCHAR 2
@@ -93,5 +94,5 @@ static inline void sbi_remote_sfence_vma_asid(const unsigned long *hart_mask,
 {
 	SBI_CALL_4(SBI_REMOTE_SFENCE_VMA_ASID, hart_mask, start, size, asid);
 }
-
-#endif
+#endif /* CONFIG_RISCV_SBI */
+#endif /* _ASM_RISCV_SBI_H */