uboot/splash: add splash screen documentation for AM62PX, J722S and AM62LX

[lucifer-9852]

• AM62PX: update stale ti-u-boot branch links, add panel-specific config
  fragment table (OLDI and DSI panels) and build commands.
• J722S: add custom board steps, splash storage struct (SPI-NOR + MMC),
  env variables, panel config fragment table (OLDI, DSI, eDP) and build
  commands.
• AM62LX: add custom board steps, splash storage struct (MMC only), env
  variables with correct DDR load address (0x82200000), panel config
  fragment table (DSI) and build commands; add warning that display
  pipeline drivers must be built-in (=y) in Linux kernel when U-Boot
  splash is enabled.
• Add UG-Splash-Screen to J722S and AM62LX TOC files.

[github-actions]

New warnings found with rstcheck:

source/linux/Foundational_Components/U-Boot/UG-Splash-Screen.rst:278: (INFO/1) Enumerated list start value not ordinal-1: "3" (ordinal 3)
source/linux/Foundational_Components/U-Boot/UG-Splash-Screen.rst:318: (INFO/1) Enumerated list start value not ordinal-1: "4" (ordinal 4)
source/linux/Foundational_Components/U-Boot/UG-Splash-Screen.rst:508: (INFO/1) Duplicate implicit target name: "panel-specific config fragments".
source/linux/Foundational_Components/U-Boot/UG-Splash-Screen.rst:554: (INFO/1) Duplicate implicit target name: "building u-boot with splash screen enabled".

[StaticRocket]

This file needs to be cleaned up at some point. No reason to interlace different steps for each machine. Additionally, splitting a list with any other content type at the same or higher indentation level actually results in 2 list elements, mangling templates.

[StaticRocket]

Considering this PR is doubling the size of this problematic section, could you look into resolving that?

[devarsht]

Isn't this already included by default in current sdk ?

[devarsht]

The above config fragment is specific to a53 spl splash.

There are below two sections in the document:

1. How to run splash in uboot proper via cmdline -> Highlight extra Kconfigs which need to be enabled, Kconfigs which need to be disabled etc.

2. How to run splash with upstream uboot -> I think we would need to hide this section for now as patches are not posted upstream

You need to document steps for above two sections too for the new platforms and interfaces which were added and until then these two sections should not give false information for j722s or the dsi interfaces for am62p, am62l, j722s.

[devarsht]

I guess we don't need to add video driver and panel node for any of the platforms now or AM62 is still following the old style?

Also for J722S and other platforms there is a direct overlay so we don't need to mention "Add video driver and panel node in the dts file"