arc_jtag_enque_read_dr() function

Read 4-byte word from data register. Unlike arc_jtag_write_data, this function returns byte-buffer, caller must convert this data to required format himself. This is done, because it is impossible to convert data before jtag_execute_queue() is invoked, so it cannot be done inside this function, so it has to operate with byte-buffers. Write function on the other hand can "write-and-forget", data is converted to byte-buffer before jtag_execute_queue().