docs(stm32mp1): fix FIP command with OP-TEE

When building a FIP with OP-TEE as BL32 on STM32MP1, AARCH32_SP=optee
has to be added to the make command.

Change-Id: I900c01957fe4ed7ed13ca955edd91ed1c5c5c4fa
Signed-off-by: Yann Gautier <yann.gautier@st.com>
diff --git a/docs/plat/stm32mp1.rst b/docs/plat/stm32mp1.rst
index 17f7a86..af302c6 100644
--- a/docs/plat/stm32mp1.rst
+++ b/docs/plat/stm32mp1.rst
@@ -181,6 +181,7 @@
 .. code:: bash
 
     make CROSS_COMPILE=arm-none-eabi- PLAT=stm32mp1 ARCH=aarch32 ARM_ARCH_MAJOR=7 \
+        AARCH32_SP=optee \
         DTB_FILE_NAME=stm32mp157c-ev1.dtb \
         BL33=<u-boot_directory>/u-boot-nodtb.bin \
         BL33_CFG=<u-boot_directory>/u-boot.dtb \