mmc: sdhci-of-aspeed: enable CONFIG_MMC_SDHCI_IO_ACCESSORS

Enable CONFIG_MMC_SDHCI_IO_ACCESSORS for the ASPEED MMC driver. The read_l
callback is used for inverted card detection.

Signed-off-by: Ivan Mikhaylov <i.mikhaylov@yadro.com>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>