feat(U_boot): add general info for the AM62Lx boot flow

Quite a lot has changed for the AM62Lx boot flow when compared to more
traditional AM62 devices. After hiding un-necessary sections document
via ASCII charts the bootflow between the SMS and A53 cores along with
the memory layout of the 2 SRAM spaces during the boot sequence

Signed-off-by: Bryan Brattlof <[email protected]>

Author: bryanbrattlof