scripts: dtc: Fetch fdtoverlay.c from external DTC project

commit 6d01cb955c53f01de53c27c350b3a2610f6fdfc4 from
git://git.ti.com/ti-linux-kernel/ti-linux-kernel.git

commit b775f49f

 upstream.

We will start building overlays for platforms soon in the kernel and
would need fdtoverlay tool going forward. Lets start fetching it.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/28f66f70602225bb6aeb58e924c20bde9d864327.1611904394.git.viresh.kumar@linaro.org
[s-anna@ti.com: cherry-pick commit 'b775f49f

' from v5.12]
Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Xulin Sun <xulin.sun@windriver.com>