irqchip: Define EXYNOS_IRQ_COMBINER

This patch is written to clean up dependency of ARCH_EXYNOS
Not all exynos device have IRQ_COMBINER, especially aarch64 EXYNOS
but it is built for all exynos devices.
Thus add the config for EXYNOS_IRQ_COMBINER
remove direct dependency between ARCH_EXYNOS and exynos-combiner.c
and only selected on the aarch32 devices

Signed-off-by: Hyunki Koo <hyunki00.koo@samsung.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20191224211108.7128-1-hyunki00.koo@gmail.com