uboot-phytec

pico-imx7d: 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: Joris Offouga <offougajoris@gmail.com>