mx6ulevk: Include SDP boot instructions in README

Add instructions on how to boot mx6ul_14x14_evk_defconfig target
using the Serial Download Protocol.

Add examples from uuu and imx_usb_loader tools.

Signed-off-by: Breno Lima <breno.lima@nxp.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>