test: cmd: fdt: Rename fdt_test_get() to fdt_test_get_value()

The 'fdt get' command has a 'get value' subcommand, rename the fdt_test_get()
to fdt_test_get_value() to avoid confusion about what it is testing. There is
currently no get 'get name', 'get addr', 'get size' subcommand test.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Simon Glass <sjg@chromium.org>