clk: ingenic: add driver for Ingenic SoC CGU clocks

clk: ingenic: add driver for Ingenic SoC CGU clocks

This driver supports the CGU clocks for Ingenic SoCs. It is generic
enough to be usable across at least the JZ4740 to the JZ4780, and will
be made use of on such devices in subsequent commits. This patch by
itself only adds the SoC-agnostic infrastructure that forms the bulk of
the CGU driver for the aforementioned further commits to make use of.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Co-authored-by: Paul Cercueil <paul@crapouillou.net>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Mike Turquette <mturquette@linaro.org>
Cc: Stephen Boyd <sboyd@codeaurora.org>
Cc: linux-clk@vger.kernel.org
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/10150/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>