mmc: Kconfig: Enable CONFIG_MMC_SDHCI_IO_ACCESSORS

Enable CONFIG_MMC_SDHCI_IO_ACCESSORS so that SDHC controller specific
register read and write APIs, if registered, can be used.

Signed-off-by: Vijay Viswanath <vviswana@codeaurora.org>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>