dts: versatile: add pl180 compatible strings

While not needed for probing, add the "arm,pl180" compatible string for
completeness.

Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Linus Walleij <linus.walleij@linaro.org>