mfd: Add irq io-resource for tps6586x rtc sub driver

Add IRQ IORESOURCE for rtc sub driver of this device.
The rtc driver can get the irq by calling platform_get_irq().

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Reviewed-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>