mmc: core: Implement ->sw_reset bus ops for SDIO

Let's implement the ->sw_reset() bus ops to allow SDIO func drivers, in
particular, to make a SW reset without doing a full power cycle of the SDIO
card.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Tested-by: Quentin Schulz <quentin.schulz@bootlin.com>
Reviewed-by: Shawn Lin <shawn.lin@rock-chips.com>