The existing bootph binding is defined such that properties in a
subnode are also implied in the supernode also, as in this example:

   buttons {
      /* bootph,pre-ram is implied by btn1 */
      compatible = "gpio-keys";

      btn1 {
         bootph,pre-ram;
         gpios = <&gpio_a 3 0>;
         label = "button1";
         linux,code = <BTN_1>;
      };

Provide an option to implement this in fdtgrep.

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