board: stemmy: Add basic Fastboot support

Make use of the new drivers for ARM U8500 introduced in the U-Boot
2021.10 merge window by adding basic support for USB Fastboot with
the "stemmy" board. As a first step this will always boot directly
into USB Fastboot for now with the console displayed on the screen
to make that obvious.

Samsung uses quite strange GPT partition labels on these boards,
so also add a bunch of fastboot_partition_alias_* to make this more
easy to use.

Signed-off-by: Stephan Gerhold <stephan@gerhold.net>