test: cmd: fdt: Add fdt get value test case

Add test case for 'fdt get value' sub command.

The test case can be triggered using:
"
./u-boot -d u-boot.dtb -c 'ut fdt'
"

Acked-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Signed-off-by: Marek Vasut <marex@denx.de>