doc: board: phytec: Add phyGATE-Tauri board documentation

Add brief documentation on how to build a bootable U-Boot image for the
phyGATE-Tauri-L.

Signed-off-by: Yannic Moog <y.moog@phytec.de>
Reviewed-by: Teresa Remmet <t.remmet@phytec.de>
3 files changed