ARM: ux500: fix compilation after local timer rework

mach-ux500/timer.c lacked the inclusion of mach/irqs.h, and thus
failed to compile. Fix it and also remove an unused variable.

Test compiled only.

Reported-by: Lee Jones <lee.jones@linaro.org>
Cc: Arnd Bergman <arnd@arndb.de>
Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>