docs(st): one device flag for ST platforms

Due to embedded SRAM used to load BL2 and BL31 or BL32 has a limited
size, only one storage device or serial device flag should be selected
in TF-A build command line for ST platforms.
This is in line with STMicroelectionics recommendation [1] about those
compilation flags.

[1]: https://wiki.st.com/stm32mpu/wiki/How_to_configure_TF-A_BL2#Build_command_details

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I6f6ab17d45d00289989a606d15c143e5710c64ce
1 file changed