mmc: pwrseq: Add reset callback to the struct mmc_pwrseq_ops

The ->reset() callback is needed to implement a better support for eMMC HW
reset. The following changes will take advantage of the new callback.

Suggested-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>