irqchip: exynos: localize irq lookup for ATAGS

The IRQ_SPI() macro is not available in the driver when building with sparse
IRQs or multiplatform, so let's move all users of this into one function
that we can leave out when building DT-only.

Signed-off-by: Arnd Bergmann <arnd@arnd.de>
Cc: Thomas Gleixner <tglx@linutronix.de>