ARM: exynos4: Fix build error due to 'gic_bank_offset' undeclared

Fix below build error:
  CC      arch/arm/mach-exynos/cpu.o
arch/arm/mach-exynos/cpu.c: In function 'exynos4_init_irq':
arch/arm/mach-exynos/cpu.c:245: error: 'gic_bank_offset' undeclared (first use in this function)
arch/arm/mach-exynos/cpu.c:245: error: (Each undeclared identifier is reported only once
arch/arm/mach-exynos/cpu.c:245: error: for each function it appears in.)
arch/arm/mach-exynos/cpu.c:243: warning: unused variable 'bank_offset'
make[1]: *** [arch/arm/mach-exynos/cpu.o] Error 1
make: *** [arch/arm/mach-exynos] Error 2

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>