mmc: host: Enable compile testing of multiple drivers

Multiple MMC host controller driver can be compile tested as they do not
depend on architecture specific headers.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200907105254.31097-1-krzk@kernel.org


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