DEVICETREE: Add Ingenic CGU binding documentation

DEVICETREE: Add Ingenic CGU binding documentation

Document the devicetree binding for Ingenic SoC CGUs, and add headers
defining the clock specifiers for clocks provided by the JZ4740 & JZ4780
CGU blocks.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Kumar Gala <galak@codeaurora.org>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mike Turquette <mturquette@linaro.org>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: devicetree@vger.kernel.org
Cc: linux-mips@linux-mips.org
Cc: devicetree@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/10152/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>