mmc: s3cmci: enable compile testing

The driver can be compile tested to increase build coverage.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20201026153810.89512-2-krzk@kernel.org


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>