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>