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".