riscv: Add cache function prototype

The function arch_clean_invalidate_caches() is required by kernel
benchmarking feature. Add the function prototype as a placeholder, not
yet implemented.

Signed-off-by: Siwei Zhuang <siwei.zhuang@data61.csiro.au>

include/arch/riscv/arch/machine/hardware.h

@@ -127,6 +127,11 @@ static inline unsigned int CONST pageBitsForSize(vm_page_size_t pagesize)
         fail("Invalid page size");
     }
 }
+
+static inline void arch_clean_invalidate_caches(void)
+{
+    /* RISC-V doesn't have an architecture defined way of flushing caches */
+}
 #endif /* __ASSEMBLER__ */
 
 #define LOAD_S STRINGIFY(LOAD)