mmc: core: Do regular power cycle when lacking eMMC HW reset support

The eMMC HW reset may be implemented either via the host ops ->hw_reset()
callback or through DT and the eMMC pwrseq. Additionally some eMMC cards
don't support HW reset.

To allow a reset to be done for the different combinations of mmc hosts
and eMMC/MMC cards, let's implement a fallback via trying a regular power
cycle. This improves the mmc block layer retry mechanism of failing I/O
requests.

Change-Id: I5cafd54327cde22ea9599543382f1b294272a42c
Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
[Ulf: Rewrote changelog]
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Git-commit: 4e6c71788d6bb0e5438fc9211fa6e52dcca01474
Git-repo: git://git.linaro.org/people/ulf.hansson/mmc.git


[vviswana@codeaurora.org: resolve trivial merge conflicts]
Signed-off-by: Vijay Viswanath <vviswana@codeaurora.org>