mmc: sdhci: Export sdhci_dumpregs

Export sdhci_dumpregs so that it can be called by drivers.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Tested-by: Ludovic Desroches <ludovic.desroches@microchip.com>