dts: versatile: add clock tree

The versatile dts is missing any clock data. Add the clocks.

It is not clear from the documentation where pclk comes from, so for
now it is a dummy clock which is sufficient for things to work.

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