ARM: EXYNOS: Remove custom init_irq callbacks

Since both exynos4_init_irq() and exynos5_init_irq() are just calling
irqchip_init(), there is no need for them to exist any more, since this
is the default that is called when init_irq callback is not specified.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>