clk: dts: document pxa clock binding

Document the device-tree binding of Marvell PXA based SoCs.
PXA clocks are mostly fixed rate and fixed ratio clocks derived from an
external oscillator, and gated by a register set (CKEN or CKEN*).

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Mike Turquette <mturquette@linaro.org>