drivers: timer: arm_arch_timer: assign a initial value to last_cycle

On FVP platform, when parameter 'bp.refcounter.use_real_time' is set
to 1, cntvct_el0 isn't count from 0 and may cause overflow issue in
first timer compare interrupt.

'bp.refcounter.use_real_time' is 0 by default.

Signed-off-by: Huifeng Zhang <Huifeng.Zhang@arm.com>