mmc: core: Convert into an enum for the poweroff-type for eMMC

mmc: core: Convert into an enum for the poweroff-type for eMMC

Currently we are only distinguishing between the suspend and the shutdown
scenarios, which make a bool sufficient as in-parameter to the various PM
functions for eMMC. However, to prepare for adding support for another
scenario in a subsequent change, let's convert into using an enum.

Suggested-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Tested-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20250407152759.25160-4-ulf.hansson@linaro.org