clk: clps711x: Add DT bindings documentation

This patch adds DT binding documentation for the Cirrus Logic
CLPS711X-based CPUs clock subsystem.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mike Turquette <mturquette@linaro.org>