fdt: Create a new OF_REAL Kconfig

The condition to indicate whether there is a runtime devicetree available
is OF_CONTROL && !OF_PLATDATA. This is a bit unweidly and is repeated in
a lot of places.

Add a new OF_REAL Kconfig which provides this information directly.

Note: This is similar in effect to LIBFDT. We might consider dropping
LIBFDT and using this instead, but this is left for now as we also have
OF_LIBFDT_OVERLAY which it would not make sense to change.

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