sandbox: Add a -T flag to use the test device tree

sandbox: Add a -T flag to use the test device tree

U-Boot already supports using -D to indicate that it should use the normal
device tree. It is sometimes useful to run with the test device tree, e.g.
when running a test. Add a -T option for this along with some
documentation.

It can be used like this:

   /tmp/b/sandbox/u-boot -T -c "ut dm pci_busdev"

(this will use /tmp/b/sandbox/arch/sandbox/dts/test.dtb as the DT)

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>