ARM: EXYNOS: Remove mach/irqs.h header

Since Exynos now uses CONFIG_SPARSE_IRQ and all remaining users of this
header has been fixed, we can safely remove it.
Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>