add new (Tilelink2) RoCC accelerator interface

Includes configuration, test programs, and documentation updates.

tests/accum.c

@@ -0,0 +1,47 @@
+#include "rocc.h"
+
+static inline void accum_write(int idx, unsigned long data)
+{
+	ROCC_INSTRUCTION_SS(0, data, idx, 0);
+}
+
+static inline unsigned long accum_read(int idx)
+{
+	unsigned long value;
+	ROCC_INSTRUCTION_DSS(0, value, 0, idx, 1);
+	return value;
+}
+
+static inline void accum_load(int idx, void *ptr)
+{
+	asm volatile ("fence");
+	ROCC_INSTRUCTION_SS(0, (uintptr_t) ptr, idx, 2);
+}
+
+static inline void accum_add(int idx, unsigned long addend)
+{
+	ROCC_INSTRUCTION_SS(0, addend, idx, 3);
+}
+
+unsigned long data = 0x3421L;
+
+int main(void)
+{
+	unsigned long result;
+
+	accum_load(0, &data);
+	accum_add(0, 2);
+	result = accum_read(0);
+
+	if (result != data + 2)
+		return 1;
+
+	accum_write(0, 3);
+	accum_add(0, 1);
+	result = accum_read(0);
+
+	if (result != 4)
+		return 2;
+
+	return 0;
+}