test: cmd: fdt: Test fdt rsvmem

test: cmd: fdt: Test fdt rsvmem

Add 'fdt rsvmem' test which works as follows:
- Create custom FDT with single reserved memory (rsvmem) entry, map it to sysmem
- Add new rsvmem entry
- Delete existing older rsvmem entry
- Add new rsvmem entry again
- Always print the rsvmem list and validate it

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>