doc: phytec: imx8mp: add OP-TEE documentation

Add documentation for the phyBOARD-Pollux i.MX 8M Plus on OP-TEE
integration.
Also add missing '-' to TF-A build instruction while at it.

Signed-off-by: Yannic Moog <[e-mail]>

doc/board/phytec/phycore-imx8mp.rst

···
 -----------
 
 - Build the ARM Trusted firmware binary
+- Build the OP-TEE binary (optional)
 - Get ddr firmware
 - Build U-Boot
 - Boot
···
 
    $ 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=0x30860000
+   $ # with optee
+   $ make PLAT=imx8mp SPD=opteed bl31
+   $ # without optee
    $ make PLAT=imx8mp bl31
 
+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-mx8mp_phyboard_pollux \
+     CFG_TZDRAM_START=0x56000000
+
 Get the ddr firmware
 --------------------
 
···
 .. code-block:: bash
 
    $ cp <TF-A dir>/build/imx8mp/release/bl31.bin .
+   $ cp <OP-TEE dir>/out/arm/core/tee-raw.bin tee.bin
    $ cp firmware-imx-8.19/firmware/ddr/synopsys/lpddr4*.bin .
 
 Build U-Boot