spl: fit: Add board level function to decide application of DTO

Add board-specific function used to indicate whether a DTO from fitImage
configuration node 'fdt' property DT and DTO list should be applied onto
the base DT or not applied.

This is useful in case of DTOs which implement e.g. different board revision
details, where such DTO should be applied on one board revision, and should
not be applied on another board revision.

Signed-off-by: Marek Vasut <marex@denx.de>