test: cmd: fdt: Test fdt mknode

Add 'fdt mknode' test which works as follows:
- Create fuller FDT, map it to sysmem
- Create node either in / or subnode
- Attempt to create node over existing node, which fails
- Attempt to create subnodes in non-existing nodes or aliases
- Verify created nodes using fdt list command

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>