binman: Add a ProcessFdt() method

binman: Add a ProcessFdt() method

Some entry types modify the device tree, e.g. to remove microcode or add a
property. So far this just modifies their local copy and does not affect
a 'shared' device tree.

Rather than doing this modification in the ObtainContents() method, and a
new ProcessFdt() method which is specifically designed to modify this
shared device tree.

Move the existing device-tree code over to use this method, reducing
ObtainContents() to the goal of just obtaining the contents without any
processing, even for device tree.

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