pico-imx7d: Increase u-boot size for dfu request

After DM conversion, the size of U-Boot binary to increase.
Previous size is 480K after DM Conversion the new size is 557K
So it's necessary to increase the dfu request for store u-boot-dtb.img in eMMC.

Signed-off-by: Joris Offouga <offougajoris@gmail.com>