stm32mp1: fix partition types to GPT format

Replace option partition-type with option partition-type-uuid as the
later is intended to GPT partitioning which is what stm32mp1 relies on.
This change prevents genimage tool, since its v13 release, from creating
a hybrid MBR partition table that is not supported by TF-A boot stage
used by the platform.

Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
Acked-by: Jerome Forissier <jerome@forissier.org>