clocksource/drivers/fttmr010: Refactor to handle clock

The plain Faraday FTTMR010 timer needs a clock to figure out its
tick rate, and the gemini reads it directly from the system
controller set-up. Split the init function and add two paths for
the two compatible-strings. We only support clocking using PCLK
because of lack of documentation on how EXTCLK works.

The Gemini still works like before, but we can also support a
generic, clock-based version.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>