doc: ti: k3: Add TIFS Stub documentation

* Add documentation to briefly explain the role of TIFS Stub in relevant
  K3 SoC's.
* Shed light on why TIFS Stub isn't package with the DM firmware itself.
* Modify the platform docs wherever the TIFS Stub documentation applies.
* Also, refactor and add a few new labels to help split the firmware
  documentation chunks. This will make it easier to include them one by
  one wherever applicable

Reviewed-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Dhruva Gole <d-gole@ti.com>
Acked-by: Francesco Dolcini <francesco.dolcini@toradex.com> # verdin-am62
diff --git a/doc/board/ti/am62px_sk.rst b/doc/board/ti/am62px_sk.rst
index c80b506..bcef665 100644
--- a/doc/board/ti/am62px_sk.rst
+++ b/doc/board/ti/am62px_sk.rst
@@ -68,6 +68,10 @@
     :start-after: .. k3_rst_include_start_boot_sources
     :end-before: .. k3_rst_include_end_boot_sources
 
+.. include::  ../ti/k3.rst
+    :start-after: .. k3_rst_include_start_boot_firmwares
+    :end-before: .. k3_rst_include_end_tifsstub
+
 Build procedure:
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