stellaris: Use the 'unimplemented' device for parts we don't implement

Use the 'unimplemented' dummy device to cover regions of the
SoC device memory map which we don't have proper device
implementations for yet.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 1484247815-15279-4-git-send-email-peter.maydell@linaro.org