cmd/fdt: add possibilty to have 'extrasize' on fdt resize

Sometimes devicetree nodes and or properties are added out of the u-boot
console, maybe through some script or manual interaction.

The devicetree as loaded or embedded is quite small, so the devicetree
has to be resized to take up those new nodes/properties.

In original the devicetree was only extended by effective
4 * add_mem_rsv.

With this commit we can add an argument to the "fdt resize" command,
which takes the extrasize to be added.

Signed-off-by: Hannes Schmelzer <hannes.schmelzer@br-automation.com>

Signed-off-by: Hannes Schmelzer <oe5hpm@oevsv.at>
Acked-by: Simon Glass <sjg@chromium.org>