fdtdec: only create phandle if caller wants it in fdtdec_add_reserved_memory()

The phandlep pointer returning the phandle to the caller is optional
and if it is not set when calling fdtdec_add_reserved_memory() it is
highly likely that the caller is not interested in a phandle to the
created reserved-memory area and really just wants that area added.

So just don't create a phandle in that case.

Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>