test: cmd: fdt: Test fdt bootcpu

Add 'fdt bootcpu' test which works as follows:
- Create basic FDT, map it to sysmem
- Print the FDT bootcpu
- Set the FDT bootcpu and read the value back using 'fdt header get'
- Perform the previous step twice to validate bootcpu overwrite

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>