ARM: S5P: Fix missed IRQ_EINT_BIT()

When merged patches, missed IRQ_EINT_BIT() definition from commit ea31fd43
(ARM: S5PV210: Add Power Management Support). The IRQ_EINT_BIT() is used
in the Power Management operation (plat-samsung/pm.c).
Signed-off-by: NKukjin Kim <kgene.kim@samsung.com>

arch/arm/plat-s5p/include/plat/irqs.h

@@ -94,6 +94,8 @@
 						((irq) - S5P_EINT_BASE1) : \
 						((irq) + 16 - S5P_EINT_BASE2))
 
+#define IRQ_EINT_BIT(x)		EINT_OFFSET(x)
+
 /* Typically only a few gpio chips require gpio interrupt support.
    To avoid memory waste irq descriptors are allocated only for
    S5P_GPIOINT_GROUP_COUNT chips, each with total number of