irqchip: exynos: allocate combiner_data dynamically

The number of combiners on a given SoC is a platform specific
constant, and we cannot encode this number on a multiplatform
kernel since the header file defining it is not available.

Allocating the structure dynamically ends up cleaner anyway
since we keep all the data local.

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