BaseTools/Conf: disable DTC legacy phandle format

By default, the device tree compiler emits phandle properties twice:
once called 'phandle' and again called 'linux,phandle'. Given that
Linux was updated in early 2010 [0] to accept the former (which is
what is specified in the ePAPR and device tree specifications), there
is no point in emitting both when compiling device trees for UEFI
platforms.

[0] 04b954a673dd02f585a2769c4945a43880faa989
"of/flattree: Make the kernel accept ePAPR style phandle information"

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>