rtc: ds1307: Convert to threaded IRQ

The driver currently emulates the concept of threaded IRQ using a
workqueue, which it really does not need to. Instead, switch over to
threaded_irq handlers which is meant precisely for the same purpose.

Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>