fastboot: blk: switch emmc to use the block helpers

fastboot: blk: switch emmc to use the block helpers

Switch the mmc backend to this new shared block helpers,
reducing block logic and only leaving MMC specific logic.

Signed-off-by: Dmitrii Merkurev <dimorinny@google.com>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@kernel.org>
Tested-by: Mattijs Korpershoek <mkorpershoek@kernel.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20251121-topic-fastboot-blk-v7-2-9589d902fc91@linaro.org
Signed-off-by: Mattijs Korpershoek <mkorpershoek@kernel.org>