doc: beagle: am62x_beagleplay: Document the use of TIFS Stub

* Include the actual common documentation about the TIFS Stub and role
  it plays to enable Low Power Modes in the platform.
* Add the AM62x boot flow to show at which point the TIFS Stub actually
  gets loaded.
* Mention the TIFS Stub in the TISPL image format.

Reviewed-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Dhruva Gole <d-gole@ti.com>