MMC:implement eMMC4.4 standard HW reset feature

Hello Chris,
These are the version 3 of HW reset feature implementation. Since I didnot get any response from the version 2 and version1 patches, I am not sure whether I need to change. So just did a little change in form.

The 2 patches implemented eMMC4.4 hardware reset feature in core layer and sdhci host layer, but need each sdhci host controller to implement the real reset part specially.

patch 0001: implemented eMMC4.4 hardware reset feature in mmc core layer. Added reinit callback in mmc_bus_ops and hardware_reset callback in mmc_host_ops.
patch 0002: implemented hardware reset callback in sdhci host layer. Added reset_emmc callback in sdhci_ops.

changelog:
patch 0001: add a error check when switch to enable HW reset bit failed. Also add some comment to explain the added 2 new callbacks. patch 0002: add some comment to explain the reset_emmc callback.

Thanks
Chuanxiao