rtc: ds1307: Switch to managed irq allocation

Since we are not doing anything fancy in remove function that requires
us to sequence IRQ free operation, we might as well switch over to devm_
equivalent of managed IRQ allocation and remove the explicit free_irq
since it'd be done automatically at remove.

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