Convert the ARM Snoop Control Unit (SCU) Device Tree binding
documentation to json-schema.

Document required properties.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/c5c36fb952675df4b9c9834d53a21fb58f391e86.1621522979.git.geert+renesas@glider.beSigned-off-by: Rob Herring <robh@kernel.org>