Add documentation for mmc_hw_reset to make sure the intended use case is
clear.
Signed-off-by: NWolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/20200918215446.65654-1-wsa+renesas@sang-engineering.comSigned-off-by: NUlf Hansson <ulf.hansson@linaro.org>