chapter2: add tfa-bl31 type

Added to U-Boot v2025.04[1].

Used for U-Boot proper to load TF-A BL31 from kernel FIT instead of the
(currently) more typical U-Boot SPL loading TF-A BL31 from U-Boot proper
FIT, allowing to run U-Boot proper in EL3.

[1] https://source.denx.de/u-boot/u-boot/-/commit/854a5a00cadd6ea4adcc9e4a923124887989f5f8

Cc: Marek Vasut <[email protected]>
Signed-off-by: Quentin Schulz <[email protected]>

Author: QSchulz

source/chapter2-source-file-format.rst

@@ -218,6 +218,7 @@ description
     sunxi_egon            Allwinner eGON Boot Image
     sunxi_toc0            Allwinner TOC0 Boot Image
     tee                   Trusted Execution Environment Image
+    tfa-bl31              Trusted Firmware-A BL31 Image
     ublimage              Davinci UBL image
     vybridimage           Vybrid Boot Image
     x86_setup             x86 setup.bin