mmc: sdhci-pltfm: use devm_request_mem_region()

Use the managed variant of request_mem_region().

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>