binman: fit: Allow running fdtgrep on devicetree blobs

binman: fit: Allow running fdtgrep on devicetree blobs

When using FIT to load firmware builds for multiple models, the FIT must
include a common binary along with a number of devicetree blobs, one for
each model. This is the same mechanism as is used for loading an OS.

However, SPL builds do not normally use the full devicetree, but instead
a cut-down version which various nodes and properties removed.

Add a new fit,fdt-phase property to allow binman to produce these
devicetree blobs.

Signed-off-by: Simon Glass <sjg@chromium.org>