arm64: dts: freescale: fix arm,sp805 compatible string

According to Documentation/devicetree/bindings/watchdog/arm,sp805.yaml
the compatible is:
  compatible = "arm,sp805", "arm,primecell";

The current compatible string doesn't exist at all. Fix it.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>