nvic: Fix ARMv7M MPU_RBAR reads

Fix an incorrect mask expression in the handling of v7M MPU_RBAR
reads that meant that we would always report the ADDR field as zero.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 1509732813-22957-1-git-send-email-peter.maydell@linaro.org