util/fifo8: implement push/pop of multiple bytes

The patch adds functions fifo8_push_all() and fifo8_pop_buf() which
can be used respectively to push the content of a memory buffer to the
fifo and to pop multiple bytes obtaining a pointer to the fifo backing
buffer.

In addition, it implements fifo8_num_free() and fifo8_num_used() which
allow to check if a multi-byte operation can be performed.

Signed-off-by: Beniamino Galvani <b.galvani@gmail.com>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
