doc: board: ti: am64: Add boot flow diagram

Add documenatation and boot flow diagram for AM64 EVM/SoC.

Suggested-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Roger Quadros <rogerq@kernel.org>
Reviewed-by: Nishanth Menon <nm@ti.com>
Tested-by: Nishanth Menon <nm@ti.com> #SK-AM64B