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>
diff --git a/doc/board/phytec/imx8mm-optee-build.rsti b/doc/board/phytec/imx8mm-optee-build.rsti
new file mode 100644
index 0000000..c83de5c
--- /dev/null
+++ b/doc/board/phytec/imx8mm-optee-build.rsti
@@ -0,0 +1,14 @@
+Build the OP-TEE binary (optional)
+----------------------------------
+
+.. code-block:: bash
+
+   $ git clone https://github.com/OP-TEE/optee_os.git
+   $ cd optee_os
+   $ make CFG_TEE_BENCHMARK=n \
+     CROSS_COMPILE=aarch64-linux-gnu- \
+     O=out/arm \
+     PLATFORM=imx-mx8mmevk \
+     CFG_TZDRAM_START=0x56000000 \
+     CFG_DDR_SIZE=0x80000000 \
+     CFG_UART_BASE=UART3_BASE