warp7: Fix boot by selecting CONFIG_OF_LIBFDT

CONFIG_OF_LIBFDT needs to be selected to avoid the following
boot problem:

reading zImage
6346216 bytes read in 118 ms (51.3 MiB/s)
Booting from mmc ...
reading imx7d-warp.dtb
32593 bytes read in 11 ms (2.8 MiB/s)
Kernel image @ 0x80800000 [ 0x000000 - 0x60d5e8 ]
FDT and ATAGS support not compiled in - hanging
### ERROR ### Please RESET the board ###

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>