clk: add ARM syscon ICST device tree bindings

This adds the device tree bindings for the ARM Syscon ICST
oscillators, which is a register-level interface to the
Integrated Device Technology (IDT) ICS525 and ICS307
serially programmable oscillators.

Cc: devicetree@vger.kernel.org
Cc: Michael Turquette <mturquette@baylibre.com>
Cc: linux-clk@vger.kernel.org
Acked-by: Stephen Boyd <sboyd@codeaurora.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>