drivers/timer/arm_arch_timer: Fix cycles overflow with GDB stub

If GDBSTUB is enabled and the kernel runs in tickless mode, the timer
must not convert the delta cycles to a 32-bit data type (cycle_diff_t in
this case). The delta_ticks variable would overflow and the next timeout
would be set before the current timestamp, thus generating an interrupt
right after leaving the handler. As a result, the system would receive
tens of thousands of interrupts per second and would not boot.

Cc: Michal Sojka <michal.sojka@cvut.cz>
Signed-off-by: Marek Vedral <marek.vedral@gmail.com>