irqchip: exynos: pass max combiner number to combiner_init

We can find out the number of combined IRQs from the device
tree, but in case of ATAGS boot, the driver currently uses
hardcoded values based on the SoC type. We can't do that
in general for a multiplatform kernel, so let's instead pass
this information from platform code directly in case of
ATAGS boot.

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