rtc: imxdi: use the security violation interrupt

The DryIce chipset has a dedicated security violation interrupt that is
triggered for security violations (if configured to do so).  According
to the publicly available imx258 reference manual, irq 56 is used for
this interrupt.

If an irq number is provided for the security violation interrupt,
install the same handler that we're already using for the "normal"
interrupt.

imxdi->irq is used only in the probe function, make it a local variable.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>