mmc: sdhci: Let drivers decide whether to use mmc_retune_needed() with pm

Devices might save and restore tuning values so that re-tuning might not be
needed after a pm transition.  Let drivers decide by pushing the
mmc_retune_needed() logic down to them.

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>