irqchip: exynos-combiner: remove hard-coded irq_base value

Replace irq_domain_add_simple with "irq_domain_add_linear" in order to use
linear irq domain, and to remove hardcoded irq_base_value.

Signed-off-by: Chander Kashyap <chander.kashyap@linaro.org>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>