stm32mp1: update doc for U-Boot compilation

U-Boot should be compiled with stm32mp15_trusted_defconfig which is
supported since tag v2019.07-rc1 with commit [1].

The creation of the U-Boot binary with stm32 header is done at U-Boot
compilation step, it is no more required to call the extra command.

[1] https://git.denx.de/?p=u-boot.git;a=commit;h=015289580f81

Change-Id: Ia875c22184785fc2e02ad07993a649069cd5ce34
Signed-off-by: Yann Gautier <yann.gautier@st.com>

docs/plat/stm32mp1.rst

@@ -83,9 +83,8 @@ To build with SP_min:
 
     make CROSS_COMPILE=arm-linux-gnueabihf- PLAT=stm32mp1 ARCH=aarch32 ARM_ARCH_MAJOR=7 AARCH32_SP=sp_min DTB_FILE_NAME=stm32mp157c-ev1.dtb
     cd <u-boot_directory>
-    make stm32mp15_basic_defconfig
+    make stm32mp15_trusted_defconfig
     make DEVICE_TREE=stm32mp157c-ev1 all
-    ./tools/mkimage -T stm32image -a 0xC0100000 -e 0xC0100000 -d u-boot.bin u-boot.stm32
 
 To build TF-A with with Op-TEE support: