riscv: dts: no default configuration for MULTI_DTB_FIT

riscv: dts: no default configuration for MULTI_DTB_FIT

JH7110 boards are currently the only use case for multi DTB FIT images
on RISC-V.

Booting JH7110 systems with a VisionFive 2 device-tree used to kind of
work without causing harm to the hardware. But there is no guarantee
that this will hold true in future. So we should not rely on it.

Before the current patch series booting failed on unsupported boards due
to the lack of a device-tree in the binman generated default configuration
when reaching main U-Boot.

By not setting a default configuration booting will now fail on
unsupported boards already in SPL. This allows SPL to
continue with the next boot source for a possible recovery.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: E Shattow <e@freeshell.de>