mmc: core: add eMMC hardware reset support

eMMC's may have a hardware reset line.  This patch provides a
host controller operation to implement hardware reset and
a function to reset and reinitialize the card.  Also, for MMC,
the reset is always performed before initialization.

The host must set the new host capability MMC_CAP_HW_RESET
to enable hardware reset.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Chris Ball <cjb@laptop.org>