doc: board: Add document for DFU boot on am62x SoCs

Both AM62 SK and beagleplay support DFU boot in a similar way now;
Document how to actually run DFU boot for both boards

Signed-off-by: Sjoerd Simons <sjoerd@collabora.com>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Tested-by: Mattijs Korpershoek <mkorpershoek@baylibre.com> # on beagle play
Signed-off-by: Martyn Welch <martyn.welch@collabora.com>