dt-bindings: clock: Convert Calxeda clock bindings to json-schema

Convert the Calxeda clock bindings to DT schema format using json-schema.

This just covers the actual PLL and divider clock nodes. In the actual
DTs they are somewhat unconnected (no ranges or bus compatible) children
of the sregs node, but for the actual clock bindings this is not
relevant.

One oddity is that the addresses are relative to the parent node,
without that being pronounced using a ranges property.
But this is too late to fix now.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>