ARM: vic: fix build warning caused by previous commit

07c9249f1f (ARM: 7554/1: VIC: use irq_domain_add_simple()) removed the
use of 'irq_base' but did not remove the variable itself, which causes:

arch/arm/common/vic.c: In function 'vic_of_init':
arch/arm/common/vic.c:410:6: warning: unused variable 'irq_base'

Remove this now unused variable.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>