powerpc/dts: fix the compatible string of sec 4.0

Fix the compatible string of sec 4.0 to match with CAAM driver according
to Documentation/devicetree/bindings/crypto/fsl-sec4.txt

Signed-off-by: Liu Shuo <shuo.liu@freescale.com>
Signed-off-by: Shengzhou Liu <Shengzhou.Liu@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>