mmc: core: document mmc_hw_reset()

Add documentation for mmc_hw_reset to make sure the intended use case is
clear.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/20200918215446.65654-1-wsa+renesas@sang-engineering.com


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>