clocksource/drivers/c-sky: Add gx6605s SOC system timer

clocksource/drivers/c-sky: Add gx6605s SOC system timer

The driver is for gx6605s SOC system timer and there are two
same timers in gx6605s. We use one for clkevt and another one for
clksrc.

The timer is mmio map to access, so we need give mmio address in dts.

The counter at 0x0  offset is clock event.
The counter at 0x40 offset is clock source.

Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>