Add a reset function to the pxa2xx_mmci device; previously it had
no handling for system reset at all.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Message-id: 1455646193-13238-11-git-send-email-peter.maydell@linaro.org