bootstd: Add bootflow_iter_check_mmc() helper

bootstd: Add bootflow_iter_check_mmc() helper

Some bootflows might be able to only boot from MMC devices.

Add a helper function these bootflows can use.

Reviewed-by: Igor Opaniuk <igor.opaniuk@gmail.com>
Reviewed-by: Julien Masson <jmasson@baylibre.com>
Reviewed-by: Guillaume La Roque <glaroque@baylibre.com>
Tested-by: Guillaume La Roque <glaroque@baylibre.com>
Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Reviewed-by: Simon Glass <sjg@chromium.org>