mfd: da9063: Get irq base dynamically before registering device

After registering mfd device with proper irq_base
platform_get_irq_byname() calls will return VIRQ instead of local IRQ.
This fixes da9063 rtc registration issue:
da9063-rtc da9063-rtc: Failed to request ALARM IRQ 1: -22

Signed-off-by: Dmitry Lavnikevich <d.lavnikevich@sam-solutions.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>