Implements FSR register, it is used for busy waits.

Signed-off-by: Marcin Krzeminski <marcin.krzeminski@nokia.com>
Reviewed-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Message-id: 1458719789-29868-10-git-send-email-marcin.krzeminski@nokia.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>