test: cmd: fdt: Test fdt resize

Add 'fdt resize' test which works as follows:
- Create simple FDT with extra size 0, map it to sysmem
- 'resize' the FDT by 0x2000 bytes
- Verify the new space has been added to the FDT

The test case can be triggered using:
"
./u-boot -Dc 'ut fdt'
"
To dump the full output from commands used during test, add '-v' flag.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Simon Glass <sjg@chromium.org>