venice2: move device tree to fix build error

Commit 5ab502cb gathered all device tree sources
to arch/$(ARCH)/dts/.
So tegra124-venice2.dts also must go to arch/arm/dts directory
to build venice2 board.

(Commit 5ab502cb had been posted before venice2 board support
was merged. So an unvisible conflict happened.)

Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@ti.com>