documentation/devicetree: Fix a typo in exynos-dw-mshc.txt

Fixed a typo in referenced file name.

Cc: Thomas Abraham <thomas.abraham@linaro.org>
Cc: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>