arm_arch_timer: properly fix cycles overflow with GDB stub

In commit 60682555 ("drivers/timer/arm_arch_timer: Fix cycles
overflow with GDB stub") an extra TO_CYCLE_DIFF() macro was introduced
to bypass the cycle_diff_t type cast in some cases.

The actual fix consists in defining that type properly in the first
place. That's the very reason why such type was abstracted.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>