clk: ingenic: Add proper Kconfig entries

Previously, the CGU code corresponding to the SoC for which we're
compiling the kernel was the only one enabled, which made it impossible
to build one kernel that supports them all.

Now, it is possible to select more than one SoC to support.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>