Convert the sd_reset() function into a proper Device reset method.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Message-id: 1455646193-13238-4-git-send-email-peter.maydell@linaro.org