ARM: dts: imx51: Add label for VPU node

Add label for VPU node so that board dts files can disable VPU
if needed.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Acked-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>