doc: ti: am62*: Mention TIFS Stub in img fmts and boot flow

Since AM62x, AM62P and AM62A all use similar boot flows and their low
power mode s/w ARCH is also similar in the way that they make use of the
TIFS Stub, update their documentation to show where TIFS Stub is.

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