env: mmc: select GPT env partition by type guid

Since commit c0364ce1 ("doc/README.gpt: define partition type GUID for
U-Boot environment"), a specific type GUID can be used to indicate
the U-Boot environment partition on the device with GPT partition table.

This patch uses this type GUID to found the env partition as fallback
when the partition name property "u-boot,mmc-env-partition" is not present
in config node or if the indicated partition name is not found.

The mmc_offset_try_partition() function is reused, it selects the first
partition with the correct type GUID when the parameter 'str' is NULL.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>