clocksource/drivers/timer-of: Use unique device name instead of timer

If a hardware-specific driver does not provide a name, the timer-of core
falls back to device_node.name.  Due to generic DT node naming policies,
that name is almost always "timer", and thus doesn't identify the actual
timer used.

Fix this by using device_node.full_name instead, which includes the unit
addrees.

Example impact on /proc/timer_list:

    -Clock Event Device: timer
    +Clock Event Device: timer@fcfec400

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20191016144747.29538-3-geert+renesas@glider.be