mmc: sdhci-pci: Let devices define how to add the host

SDHCI provides more flexibility than simply calling sdhci_add_host(). Make
that available by allowing devices to specify their own ->add_host()
function.

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>