add new (Tilelink2) RoCC accelerator interface

Includes configuration, test programs, and documentation updates.

tests/charcount.c

@@ -0,0 +1,19 @@
+#include "rocc.h"
+
+char string[64] = "The quick brown fox jumped over the lazy dog";
+
+static inline unsigned long count_chars(char *start, char needle)
+{
+	unsigned long count;
+	asm volatile ("fence");
+	ROCC_INSTRUCTION_DSS(2, count, start, needle, 0);
+	return count;
+}
+
+int main(void)
+{
+	unsigned long count = count_chars(string + 14, 'o');
+	if (count != 3)
+		return count + 1;
+	return 0;
+}