uboot-phytec

pico-imx6ul: README: Adjust the binary name after DM conversion

After the conversion to DM the U-Boot binary is called u-boot-dtb.imx,
so fix the README file accordingly.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Acked-by: Otavio Salvador <otavio@ossystems.com.br>