starfive: fix default FDT file location
In the Linux kernel source, the FDT is named "starfive/jh7100-starfive-visionfive-v1.dtb".
Fix DEFAULT_FDT_FILE accordingly, this is necessary when using "fdtdir" in extlinux.conf.
See https://github.com/starfive-tech/linux/tree/visionfive/arch/riscv/boot/dts/starfive
Signed-off-by: Baptiste Jonglez <git@bitsofnetworks.org>