mmc: Checks EXT_CSD_PARTITION_SETTING_COMPLETED before partitions computation

Checks EXT_CSD_PARTITION_SETTING_COMPLETED bit before
 computing enhanced user area offset and size, and
 adding mmc general purpose partitions. The two needs
 EXT_CSD_PARTITION_SETTING_COMPLETED bit be set to be
 valid (as described in JEDEC standard).
Warn user in case of misconfiguration.

Signed-off-by: Grégory Soutadé <gsoutade@neotion.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>