clocksource/drivers/moxart: Use struct to hold state

Add a struct moxart_timer to hold the driver state, including the
irqaction and struct clock_event_device.

Most importantly this holds values for enabling and disabling the timer,
so future support can be added for devices that use different bits for
enable/disable.

In preparation for future hardware support we add a MOXART prefix to the
existing values.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>