doc: phytec: imx8mm: add OP-TEE documentation

Add instructions on how to build and package OP-TEE for the
phycore-imx8mm based boards. The build instructions are identical for
phyGATE-Tauri-L and phyBOARD-Polis.
Also fix missig '-' for TF-A build instructions.

Signed-off-by: Yannic Moog <y.moog@phytec.de>
4 files changed