test: cmd: fdt: Test fdt set

Add 'fdt set' test which works as follows:
- Create fuller FDT, map it to sysmem
- Set either existing property to overwrite it, or new property
- Test setting both single properties as well as string and integer arrays
- Test setting to non-existent nodes and aliases
- Verify set values using 'fdt get value'

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>