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/phycore-imx8mm.rst b/doc/board/phytec/phycore-imx8mm.rst
index e9dc225..08f5949 100644
--- a/doc/board/phytec/phycore-imx8mm.rst
+++ b/doc/board/phytec/phycore-imx8mm.rst
@@ -9,6 +9,7 @@
 -----------
 
 - Build the ARM Trusted firmware binary
+- Build the OP-TEE binary (optional)
 - Get ddr firmware
 - Build U-Boot
 - Boot
@@ -20,10 +21,15 @@
 
    $ git clone https://git.trustedfirmware.org/TF-A/trusted-firmware-a.git
    $ cd trusted-firmware-a
-   $ export CROSS_COMPILE=aarch64-linux-gnu
+   $ export CROSS_COMPILE=aarch64-linux-gnu-
    $ export IMX_BOOT_UART_BASE=0x30880000
+   $ # with optee
+   $ make PLAT=imx8mm BL32_BASE=0x56000000 SPD=opteed bl31
+   $ # without optee
    $ make PLAT=imx8mm bl31
 
+.. include:: imx8mm-optee-build.rsti
+
 Get the ddr firmware
 --------------------