mmc: sdhci-of-arasan: Fix non static symbol warning

Fixes the following sparse warning:

drivers/mmc/host/sdhci-of-arasan.c:253:6: warning:
 symbol 'sdhci_arasan_reset' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Sören Brinkmann <soren.brinkmann@xilinx.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>